Ok, I'll probably go for games that I have spent most time on, and have come back to again and again.
4X:
Civilization I (Amiga) - I got so bad GCSEs because of this game.
Civilization III (PC) - Didn't like II, not sure why, just thought it was a little repetative. III I like, but only really play it with mods (esp. DyP).
Master of Orion I (PC) - Absolute classic. I think I got more angry at this game than any other. "What?!? A 100 ship fleet on my doorstep in turn 5". I guess the AI wasn't up to much, it got _huge_ bonuses.
RTS:
Dune (PC) - Not much of a RTS fan, but liked this.
RPG:
Angband, Zangband, TOME (PC) - I always come back to these, predominantely Zangband. There is just so much variety.
Betrayal at Krondor (PC) - I remember playing this with a friend, we never finished it, but it was classic fun.
Baldur's Gate II (PC) - Still going through this, it's massive and enjoyable all the way.
Driving:
Gran Turismo series (PS1 and PS2) - Can't be beat.
V-Rally I (PS1) - Sheer driving 1v1 fun. I used to play this all the time at university.
Demolition Derby I (PS1) - An oldie, simple, but a goodie.
Supercars I & II (Amiga) - Great little arcade racers. Homing missiles were ace.
FPS:
DOOM I and II (PC) - These introduced me to multiplayer gaming. We had a 4 computers network at home, and we had day long parties which were intended to be DOOM during the day, other stuff in the evening. DOOM took over all.
Counterstrike (PC) - I actually got pretty good at this, and it was amazingly great Online. Hacks killed it a bit in some cases, but mostly it was great fun.
Quake III (PC): Great fun Online, railgun fanatics of the world unite.
Sports:
Sensible Soccer (Amiga & PC) - I just loved this game, but the computer wasn't much of a challenge, and none of my mates would play me because I just whipped them. It was silly fun though.
Tracksuit Manager (C64) - No idea why I got hooked on this, but I did. It was crap, but I did win the world cup once.
Speedball II (Amiga) - So much fun. I spent about 6 hours playing this a couple of nights ago, with an emulator.
Decathlon (C64) - Nothing like a frenzy of joystick waggling to get the adrenaline up. I think I got over 11000 points once.
Tony Hawkes Pro Skating II & III(PS1 & PS2) - I am generally a pretty good gamer, but these I am _so_ bad at. I enjoy them lots though.
SSX (PS2) - Fun game, I wasn't too bad at it either, unlike the previous.
Space:
X2 - The Threat (PC) - The game I'm currently playing most at the moment, especially now the 1.4 patch is out. Immense scope, great game. Storyline sucks though, but that's not the point to the game.
Frontier (Amiga) - Liked, but didn't love this game. Too many glitches, it was pretty huge and limitless though, and kept me playing for hours and hours.
Other:
Pirates! (Amiga) - I never did find my sister, but did just about everything else. I basically owned that Carribean by the end.
Railroad Tycoon I (Amiga) - I played this game so much. II and III didn't capture my imagination as much, but I still played them both pretty much (III still). III's landscaping is so annoying though.
Grand Theft Auto Series (Amiga & PC) - The original 2 were a bLast, I loved unnecessary killing even back then
. III made it prettier, and kept all the fun elements.
Worms (PS1) - So much time went on this game. If Civ I killed my GCSEs, this game killed my degree. So many times I was doing work, and a friend came round and asked "Fancy a game of Worms?". By the end, we had played literally thousands of games, each best of 3 rounds. The scoreboard was all screwed up, because we'd played it so much.
I've been playing with Morrowind a bit recently too, but can't seem to find any love for it. Managing stats is a bit of a pain, and there are huge numbers of exploits (Alchemy = Immediate level 10 rich character). Anyway, that's about me done, I think. I'm sure I'll think of some more later
.