6. Portal 2 – PC, PS3 and Xbox 360
Despite being blown to smithereens at the end of the first game, it seems GlaDOS is indeed still alive. Portal 2 finds Chell in the Aperture Science Enrichment Center, where she's put through further test chambers with her trusty portal gun. While the first Portal was over in four or five hours, the sequel looks likely to be a much more comprehensive experience. The addition of a co-operative campaign, where you play as hapless robots Atlas and P-body, allows a further few hours of co-operative play, and also means you can have up to four portals in any environment at one time. The potential for innovative puzzles is – even more so than its GOTY-nominated predecessor – vast.