There's certainly a time and a place for playing with yourself, but as a general rule of thumb it's always more fun to play with somebody else - or multiple people, if you're feeling particularly adventurous. We've all had a bash at some co-op gaming in the past, but here's a shiny, updated list of our Top 10 Co-Op Games of all Time.
It might not have Master Chief, but Halo: Reach has a place for almost all of your mates. You can play the campaign (with the all-important scoring, so you can prove you're better than your supposed chums), multiplayer, and ODST's returning Firefight mode - now with more options than ever. If you're looking for something more creative, you can even tool around in Forge mode and make custom levels and game modes. Microsoft is also releasing updated versions of classic maps for Reach as part of the upcoming Halo: Combat Evolved Anniversary, so there's plenty of life left in the game.