1) Morrowind - This is the last game I can remember playing that really knocked my socks off. I'd never seen anything of that scale before and my nerdy-ass love of fictional realities led me to get totally absorbed in the lore. There's a month of my life where I can't really remember anything except that I played Morrowind. A lot.
2)Deus Ex - I was initially intrigued by this game because at the time it was introduced to me, I was playing a paper n' pencil RPG called Rifts, and Deus Ex seemed to be video game version of that. Also, this game sports some of the best level design I've ever seen and was full of subjects and thoughts that were stirring around in my brain long after I'd finished the game itself. "God was a dream of good government."
3)Thief 2 - More superb level design. Some really creative mission objectives too. I don't think I was ever as obsessed with this game as I was with it's predecessor, but I cannot deny that Thief 2 had simply been improved in almost every way.
4)Hitman: Blood Money - Great level design - bit of a trend here. I also just loved the thrill of discovering new ways to murder people and get around the intricate stages. And let's face it, being 47 is just cool.
5)Civilization 4 - Man, I suck at strategy games, but I do love them so. This one is my favorite because, well.................okay I haven't analyzed my love of this series that well. Sure is good, though.