Portal 2: A remarkable sequel to a great first game. Really enjoyed it, superb visuals, puzzles and voice acting.
Batman: Arkham Asylum: A great mix of Batman, fighting and stealth. Only downside being relativley rubbish Boss Battles, the rest of it was quality.
Medieval 2: Total War: Along with the expansion, this was the pinnacle of the Total War games for me. Whilst Rome was great, this one was the highlight, mainly since I was studying the time period it covered in History at the time.
Plants vs Zombies: An addictive, polished and simple game, it's eaten enough hours of my life to deserve its place here.
Timesplitters 2: Fond memories of playing this on my Gamecube after school with pals makes this one a timeless classic.
Honourable mentions (Not to be counted): Mass Effect 2, Super Smash Bros Melee, Assassin's Creed 2, Half-Life 2