Either The Witcher 2 or Dragon Age 2; think The Witcher 2 edges it due to the sheer amount of influence the player holds over the story.
The Witcher 2 is pretty much the RPG I've wanted to play for a long time. Excellent, well-written story with characters you end up caring about. Fast paced, real-time combat on a large variety of monsters. Excellent set-pieces, without losing the depth hidden within the RPG mechanics; I'd have to nit-pick to find what I'd consider to be massive flaws in the game. Doesn't hurt it's possibly the best looking game around either.
I know the typical complaints about Dragon Age 2 - the repeated content, the waves of enemies and so on - none of that really bothered me; the quests were interesting, the combat fun and the tone - light hearted [due to the fact I chose the central dialogue option at all times], but serious enough when it needed to be kept me engaged, whilst in most Bioware games I end up skipping sections of dialogue. I think I ended up caring about the characters in DA2 more-so than even TW2 - they're all well-written with human-enough traits and flaws to make them seem somewhat realistic, despite the fantasy setting, in perhaps a way The Witcher 2 didn't quite succeed beyond its core cast. I guess it doesn't hurt I went in with the knowledge the internet generally hated the game.