The Witcher 3 and Dragon Age:Origins(+Awakening and Witch Hunt) are tied in the 1st spot for me
After them Kotor 2 with the Extended Content mod, one of the deepest games i have ever played and probably the only rappresentation of Star Wars that i actually enjoyed (Kotor 1 was too much black vs white for me, Kotor 2 is all about grey..... or purple, i guess)
Mass Effect 1 and 2, unfortunately, i don't enjoy as much anymore because of the mess that is ME3, but they are still amongst the games i most replayed back in the days
After those it's probably AC3, Planescape:Torment (that i played last year and never will replay again because it was just so great of a story), Shadowrun, Witcher 2 and that's probably it for the ones that i really loved