In No Order:
Jade Empire - I don't know why but I fell in love with this game. I've beaten it four times over, and got halfway through it once. It's just purely awesome, despite the lame ending.
Metal Gear Solid Twin Snakes - I bought the Gamecube remake, and it was a great game. Me and friend each had copies and we would play it and then talk about it, it was just one of those games that you really get into.
Warcraft 3/TFT - I've loved this game since it came ou t (playing it on and off, with the occasionaly year long break). I really only started modding it about a year ago, and I love it.
WoW - Not so much anymore, and maybe it was because I was young when I started playing it, but when I did it astounded me. I began as a dwarf (that's what my main is FYI) and the world just had this feeling to it that made it so attractive and fun to play. When I started it out the game was more of an experience than anything else. Slowly, as I've entered 'end game' I've become desensitized by it, but it always gets me when I get to explore a new area. Hell, I could go on for a month about this so.... yeah.
Oblivion - Ah, oblivion. What wasted potential. It did so many things right, yet so many things wrong. It's on my favorites not so much for what it is but for what it could be. They did so many things right and so well. The storytelling, the combat, the item system, the arena, the atmosphere. But there were these little things that always bugged me (HUGE lack of outdoor gameplay, about 5 voice actors, pretty pathetic AI, and no boss fights).