Rock Band 3!
No, wait! Let me explain!
Every year in the UK we've had a numbered edition of the Rock Band series, and every year I've absolutely loved it. But every year, another game has come out that has just narrowly beaten it (2008: Saint's Row 2, 2009: Uncharted 2). But in 2010, nothing has come close to it. Rock Band has almost been perfected (the "find online players" system is, in my mind, a step back compared to the one in RB2) and I've been loving every single second of it. Plus, I held my first Rock Band party this year and had the most fun I've ever had with any game since 2005 (my first Buzz! all-nighter if you must know).
So, yeah. Call me bias. I don't give a shit. It's my number 1. Followed closely behind by Heavy Rain (despite its faults, I still love the hell out of it).