The adjective "stunning" is over-used, especially when it comes to video game criticism. Everything's stunning these days; graphics, story, characters... everything. Well, in the case of Mass Effect 2, it's the only word that even comes close to describing the experience on offer.

In Mass Effect 2, everything is stunning. The sophisticated sci-fi story is stunning at every turn. The refined, third-person cover-based combat is stunning at the turn of every corner. And the cinematic dialogue system, voice acting and facial animations are stunning in every page-turning cut scene. It all comes together to form what may well be 2010's best game, and possibly the greatest role-playing game of all time.

Mass Effect 2's brilliance shows Canadian developer BioWare is in top form. Confidence breeds confidence, and Mass Effect 2 is one of the most confident, brash, even arrogant - but in the good way people admire - games ever created. It is a masterpiece from a developer that knows it's ahead of the game in almost every department that matters, from graphics to branching storylines to... well... everything.

Taken in isolation, Mass Effect 2 stands head and shoulders above its peers. But those who played the first game will find even more to admire, because it improves not only the things that didn't work, but the things that quite clearly did. We all know about the changes that have been made to the combat - every game magazine and website, including this one, has been banging on about them for months. Only now, though, having pumped over 40 hours into this dark, second act, do we know the true extent of BioWare's commitment to bettering what's gone before.

Firstly, let's go through the obvious stuff. The cumbersome inventory system has been streamlined to the point where you don't actually have an inventory at all. Instead, you have a base set of weapons - shared by all party members - that can be upgraded through scientific research in the Normandy tech lab. Then, before embarking on a mission, you're given the opportunity to set your loadout. It's simple and effortless, like yoghurt.

The simple act of shooting feels so much better. Headshots do extra damage, blown away limbs force Geth to a crawl, and the pistol is no longer the ultimate weapon of doom.

Further upgrades - biotic bonuses, damage multipliers and shield improvements - are all automatically applied once they're researched, whereas in the first game they had to be manually applied to each party member. Ammo types are now tech powers, and need only be triggered once per mission per gun. It's a philosophy that extends to all of Mass Effect's fluff - the fat has been cut away and dumped, leaving a lean and trim RPG that never frustrates. Ever.

Mass Effect's elevators - clever cover for extended loading - have been removed in favour of proper loading screens which show you the journey you're actually making, whether it's travelling from engineering to the captain's quarters in the Normandy, or taking a taxi ride from one part of Illium, the lawless Asari colony, to another. We quite liked Mass Effect's elevators, but not enough to rue their expiration. In any case, party members now banter as you explore Mass Effect 2's many wonderfully realised worlds, and you can get the news from kiosks and PAs.

Perhaps the main gripe with Mass Effect was the lack of a sense of exploration; off the beaten track quests were repetitive and recycled the same structures and environments over and over again. In Mass Effect 2, you certainly feel as if you can explore more of the galaxy, even if the number of planets you can actually land on has been reduced. Now, the wonderful Galaxy Map has been refined to allow you to manually fly the Normandy from system to system, consuming fuel along the way. When you arrive at a planet, you can scan it by moving a reticule slowly around its surface. Not only is this the main resource gathering mechanic - itself a strangely addictive and engrossing mini-game (one that held us in a trance-like state for over ten hours) - but impromptu side missions also reveal themselves here. These side missions are all different in some way, be it in the background to the mission, the building it takes place in, or the enemy you're up against. Some don't even require combat.

Another benefit to this new system is that the Mako Rover - so reviled by fans of the first game - is nowhere to be found. Day one DLC promises driving of some sort, but the truth is that the Mako is gone for good (we're not sorry to see the back of it). Should we have expected a galaxy full of planets on which you could land and explore? Of course not. But until we actually played the game for ourselves, we dared to dream. This is BioWare, after all.

There are many other improvements, more than we can list. The cutscenes are more dynamic, edited breathlessly in the MTV Generation style and brilliantly complemented by the new Interrupt system. The frame rate is locked at 30FPS. Textures hardly ever pop in, and there's no tearing. The new and improved Normandy is a bigger and busier place, spread out over four floors. The classes are now more defined, and feel remarkably different. Squad control is about as good as it's ever been in a video game - the contextual d-pad system directing party members as if they were hardwired to your brain. The cover system, which now involves a button press, is only bettered by that of Uncharted 2 and Gears of War 2. The graphics - a jaw-dropping fusion of Star Trek lens flare, early 80s sci-fi and the best god-damn vistas your eyes ever did see - are almost too good to be true. And the new hacking mini-games are even - shock horror - fun.

If that were it - if Mass Effect 2 was merely a game packed with improvements to areas where they weren't even needed - we'd feel perfectly satisfied. Everything that was great about the still impressive first game is just as great in the sequel; we've even started taking many of Mass Effect's qualities for granted. The third-person dialogue system, which means you're never sure how Shepard will play out your selected course of action, is a fun game in of itself, but you don't appreciate its nuances until you compare it with first-person dialogue systems in the likes of Fallout 3 and Dragon Age: Origins. The sheer sci-fi cool of it all - from the Vangelis-esque soundtrack to the power wheel "thoom", makes Mass Effect just as much of a geek love in as JJ Abrams' recent Star Trek reboot. The star-studded cast and superb voice-acting is up there with the standard bearers: Uncharted 2 and Grand Theft Auto 4. But that's not enough for BioWare. BioWare has done all of that, and innovated.

Martin Sheen's chain-smoking The Illusive Man, and Mass Effect 2's sophisticated narrative, keeps you guessing until the bitter end.

The Paragon/Renegade morality system returns, and again it's easy to know what course of action will result in Paragon or Renegade points, but the ramifications of your decisions don't reveal themselves until the game's heart-pounding climax, when Shepard and his or her eclectic team of disturbed soldiers take the fight to the Collectors - Mass Effect 2's evil alien race. You'd be forgiven for forgetting about the big decisions you've made as you near the game's thrilling end, but by the time all is said and done, you won't be able to forget them.

Some familiar faces return, but the new party members steal the show.

And BioWare's own good work with Dragon Age: Origins has clearly had an impact on Mass Effect 2's development. The new loyalty system ensures you're keeping one eye on your party members as you use the other to do battle with the Geth and the many mercenary gangs that you come up against. The game doesn't force you to spend time and effort trying to earn the respect of your party members, but you get the impression that it'll be in your best interest to do so. Shepard's suicide mission, as BioWare has explained in the run up to release, is one the Commander may not survive. Keeping everyone sweet will at the very least improve your chances.

The loyalty system manifests itself through some of the most memorable quests we've ever experienced in a role-playing game. Each of the ten potential party members has their own unique quest, which, if completed in a certain way, will make them loyal to your cause. These quests make up many of the game's best moments: they're varied, thought provoking, and seriously impact the story. Asari biotic Samara - she of the eye-popping breasts - has perhaps the best one, and made us literally scream out in astonishment at what it asked us to do. Running it a close second is Subject Zero/Jack's return to her childhood home, which sees her face her demons in a terrifying nod to BioShock's best moments. Each loyalty quest is worth your time, not just because you know it'll help you in the long run, but because they're role-playing gold.

Secondly, your team isn't just a bunch of docile automatons that merely exist in isolation on the Normandy. They're interesting, complex, and they talk to each other. At one point we arrived back on the ship to find Cerberus agent Miranda Lawson fighting the tattooed Jack. If we hadn't had our Renegade conversation option available, who knows how that one would have ended? In Mass Effect 2, there are so many resolutions to each dispute that it's seems impossible to replay the game enough to see them all.

Critics might point to Mass Effect 2's structure as evidence of a more linear experience. And indeed you can play the game one clearly signposted mission at a time, with little exploration in between. But that does a disservice to the frenetic, relentless blockbuster pace. Mass Effect 2 takes its pacing cues from the likes of Gears of War 2 and Uncharted 2; the missions tie together seamlessly and never outlast their welcome. But there's nothing stopping you from going anywhere at any point, if you want to. You can play Mass Effect 2 and finish it without having to put 40 hours in. That option is an important one, in this time-starved, socially networked age.

In any case, like the most thrilling rollercoasters, once you've finished the game the first thing you'll do is play it through again. Why? Because any single playthrough only offers a snap shot of Mass Effect 2's gargantuan innards. Try a different class - the Adept, perhaps. Try playing Paragon instead of Renegade. Make different decisions at key junctures. Try taking on the Collectors with only the minimum team requirement, and with only a few of them loyal to your galaxy-saving machinations. Try all these things in different combinations; Mass Effect 2 will adapt and tell a different story each time.

Having finished the game ourselves, we're almost overwhelmed by the sheer number of standout moments. The first time you walk towards the Afterlife nightclub on Omega and marvel at the flying car-filled night sky; the first time you gawp at the busy backgrounds of Illium, or the war-torn rubble of the Krogan homeworld Tuchanka; the moment you finally convince Jack to let her guard down and open the door to her bed; the moment Shepard interrupts a female journalist mid-question by punching her in the head; the Salarian video game sales assistant who references old school role-playing games; the game's thrilling Dead Space-esque opening, in which the Normandy is obliterated and Shepard "dies". And what about the sex scenes; not quite good enough to convince but as good an effort as we've seen in video game land so far?

Best of all, though, are the party members themselves. The shaven-headed Angelina Jolie look-a-like Jack is our favourite; her incessant swearing and voluptuous lips made us weak at the knees. The religious Assassin Thane is another stand out NPC; his mysterious motivations and keen eye with a sniper rifle make him as cool as he is deadly. But what about the sexy Miranda, or the hyper Mordin, both stars in their own right? It would have been an easy win for BioWare to simply repopulate Shepard's party with familiar faces. We're glad it didn't: Mass Effect 2's NPCs are up there with the best of all time.

Mass Effect 2 is easily a 10. Easily. We're trying desperately to think of something to moan about, just so this review doesn't sound too much like a Mass Effect love-in, but we can't. The side missions are a bit abrupt. The game still takes too long to load. And your party members will occasionally get in your line of sight. But these annoyances are as inconsequential as a fly landing on a whale's back. Mass Effect 2 is the perfect fusion of the shooter and RPG genres. When you're not playing it, you wish you were. When you are playing it, you can't imagine doing anything else. Engrossing, captivating, stunning. Roll on DLC, and Mass Effect 3.