Dark Souls - it's almost everything i want in an rpg. tough, fair, and rewarding. good amount of exploration and great atmosphere. this and demons souls felt like unintentional survivor horror games and i love it. and i'm kind of fascinated with demons and all that so any game with that theme has an easy time impressing me.
Ultimate/Marvel vs Capcom 3 - this actually isn't my favorite fighter of the generation. i can't really play it aside from just practicing combos in training mode (which is fun. combos are crazy). but this has to be the best spectator game of all time. this game got me "into" the fgc (11er here, not even an 09er lol). i have been playing fighters since the 90s, but i didn't know there was anything like the fgc (i have heard of evo, but i just thought that was it). now i'm watching weeklies, podcasts, ultrachen, lijoe...all that. while it's not my favorite fighter, marvel got me into a whole nother world.
Fallout New Vegas - i don't have too many words as to why this is a favorite, but it is. i find myself doing a playthrough each year. something i do not ever do anymore.
honorable mentions
Demon Souls - if not for Dark Souls, this would be in its place.
Borderlands 2 - it's been a week but holy hell i'm having so much fun
Portal 1/2
The Witcher 2
Street Fighter 4 series
Red Dead Redemption
Xenoblade