While not my favourite game, I'm going to say Portal, because I can't criticize any point about it, and I liked it a lot. Some games that I like more all have negative points, for instance:
Baldur's Gate 2: Not very well balanced, partially ugly graphics and steep learning curve.
Fallout 3: Oh the bugs and crashes, they almost ruined the game for me.
Diablo 2: Repetitive on a whole new scale, still fun though. Also, too many clicks!
Mass Effect 2: Incredibly boring mining mini-game, too simplified in weapons and skills.