hucklebarry: Thief. Always Thief.
I hated the abysmal ragdol physics and clunky controls of the 3rd game (not to mention the rope arrow removal and load zones due to the inferior xbox specs)... but the game was still a great Thiefy game set in that wonderful atmosphere.
Thief 4 introduced new problems and sold out in various ways that I don't approve of, but in the end, the gameplay was wonderful.
As of now, I still am utterly in love with the franchise. My biggest concern, should they do another game, is the plot. The plot in Thief 4 ramped up to pretty much nothing, which was a ginormous letdown. But due to the fun gameplay and atmosphere, I still clocked over 100 hours in that game alone.
Runners up would be Morrowind, Oblivion, Deus Ex 1, NOLF.
Thief is awesome. And you, you amazing person, just simply gave me a brand new preorder copy of Thief 4. I've just upgraded to a 64-bit system and have Thief 4 cued up as my main game again. I took a break after roughly 25 hours or so from that accursed asylum level. I couldn't play it for 10 minutes without having a heart attack. The atmosphere is so spectacular and the insane things going on just kept freaking me out. I hope to hit 100 hours with it eventually, too! There's so much to do. After I finish this play-through, I intend to get through it on a super-crazy-hard difficulty. Should be fun!
Thief 1 is definitely one of those awesome games for me. I first played a pirated copy at my college. Where I went, we had a guy that would host a few games. The Internets were still young and speeds were slow, but on our massive LAN that connected roughly 1200 students, you could move files with lightning speed. Nobody had even thought much about the morality of pirating. It was more about the possibility of sharing games. The normal cycle was to use the pirated games as demos (they often weren't complete anyway) and then buy what you loved. So I downloaded Thief b/c I loved playing thieves in RPGs. The game was missing all of the movie files, so the story made no sense. I played the tutorial level, then ran to the mall and picked up a copy. It was that good. And I just sucked up that whole game.
Might and Magic Book 2 is also huge on my list. I spent more than a decade playing it. I learned all there was to learn about it. I just liked it for its simplicity and complexity.
And lastly, Neverwinter Nights. No game has taken more hours of my life. And they were well-spent. Adventures with others or myself, the game is filled with spectacular events and, with a good DM, amazing stories that I'll always remember. Things as simple as tomb-raiding and as complex as dethroning a king. It was just an amazing game that bridged my bedroom to the rest of the world after moving half-way across the country for a 6-month stint. It was pretty neat.
Thems is the bigguns fer me.