I have read the books, and OF COURSE it's Yen:
1) The Last Wish is my singe favourite side quest in TW3.
2) In blood of elves Geralt admits to Triss that he is THAT much in love with Yen.
3) In lady of the lake Yen indirectly admits that is THAT much in love with Geralt
4) Sure, Yen is an ultra bitch on the surface, but only there. Her temper is quite interesting, while Triss IMO is a bit boring, even though, even in the books, it seems like she is the only sorceress who is indeed a nice person
5) It's, like it or not, what Geralt really wants, considering he recovered his memory.
edit:6) Far better sex scenes.