Just try. It is SO MUCH more fun!

I play loads of pre-2000 games, including various adventure games (Monkey Island, King's Quest, Space Quest, Quest for Glory and the like) RPGs (Baldurs Gate, Ultima, etc.) and strategy games (Command and Conquer, Dune II, etc.).
Currently playing "The Dig" for example. Have also reviewed SimLife and ShadowCaster for Abandonia aswell.

Playing thief; when I am finished its off to thief 2

Aye.
But I'm keen to dig out my Goldbox series from SSI.
Karl

It'll be pretty hard to narrow that down to one game. Here's a few of the ones I keep returning to:
Wizball (C64)
Moonstone (Amiga)
Monkey Island 1+2
Day Of The Tentacle
Dungeon Keeper (except that I can't play it on Vista, AT ALL)

Starquake, Oh Mummy! and plenty of other games I used to have on the CPC.

Oh, my! How could I forget Star Control 2 :-O

Majesty: the Fantasy Kingdom is probably the only game I still get out once a year to replay.

TESII Daggerfall, Magic Carpet
...and Master of Orion (when I feel the urge to prove to myself that I still suck at strategies :) )

I'd be another one for Wizball.
Story based ones, especially certain RPGs and Adventure games I need to wait on a fair while ( a couple of years), otherwise it's to familiar. If it's just a grindfest, then sure, why not eh?
Actually looking at my shelf full of CDs/DVDs. It's surprisingly hard, I'd have no issues replaying any of them, but I'm to familiar with most to bother much.

already mentioned fallout as well as Planescape torment (the backgrounds. they are frigging beautiful and will always be...you need a mod to enjoy them tough)
Master of Orion 2
HL1 after a decade, after a sequel, after thousands of other games it is still one of the best fps out there.
Transport Tycoon. Jesus. game is as much fun as 15 years ago.
Civilization 2.
Nox
Red Alert 1. sure the gameplay is primitive but it is so much fun!
Adventure games. They don't really age. Teenagent, Beneath a Steel Sky, Monkey Island, Grim Fandango, the Dig, Indiana Jones, Longest Journey, broken sword, etc.. the time passed didn't really change how much fun those games are.

There are some really good examples here, but I am suprised no one has mentioned Tetris yet. I still play Tetris DX on the Game Boy Color and it still manages to make my thumb ache like hell.

Someone mentioned Abandonia and I saw The Bard's Tale there. That's awesome. I cn't use my original disks because they are disks and I don't have a disk drive.
I'd definitely play The Bard's Tale again.
Karl

Bomberman is epic win. So many good times...