I try and put either Heroes of Might and Magic 2 or 3 on every one of my computers, except the old laptop I use for testing random software and Linux distros.

Mostly games with infinite replayability like Age of Empires or Age of Wonders. Sometimes I just want to have a 30-60 minute skirmish, and those games are still the best to do one.

Frozen Synapse!

I also can't say no to Pac-Man or Rollercoaster Tycoon.

Mostly big open world RPGs... always Morrowind, usually Fallout 3 and New Vegas, now-a-days Skyrim. I guess that's all Bethesda stuff, hmmmm.

Also FEAR and Crysis, the two best shooters ever (shut the fuck up!).

Many games I leave hanging on older PCs because I am unsure if I still want to play them a bit more (e.g. Diablo 2 + LoD on my work laptop, finished it once, but unsure if I want to continue playing it on harder difficulty, or restart with another class; or Carmageddon 2, finished it already, but sometimes I may feel like revisiting the earlier maps with my now overpowered car)...

...but the only game that surely finds its way ASAP to my new computers too, or if I reinstall Windows, is TeamFortress Classic. Even though I don't play it that much anymore, but sometimes I might get an urge.

Sometimes I saw myself installing Starcraft + Brood Wars, Age of Empires 1-2 and Age of Mythology quite often, but even them I haven't yet re-installed. Maybe just so that I don't get stuck in replaying them, again.

I have a rather large games drive (1.5 TB), so I have a ton of games installed. Right now between 85 and 90. I just don't bother uninstalling them, so I have pretty much whatever I feel like at my fingertips.

If I were forced to cull it down to just a half dozen or so, it would be:

Baldur's Gate series
Icewind Dale series
Probably the Fallout games
Neverwinter Nights
Probably Hearts of Iron 2 or something in that genre as well.

Morrowind, Baldur's Gate, Fallout, HOMM3, DOOM, and a huge stack of old DOS games I keep in a big folder together with DOSBox.

I always carry these two games over:
Mass Effect 1
Mass Effect 2

One never know when the Shepard's fever will spread again :).

Minesweeper

Civilization 4 + both AddOns

Installed on each of my 3 computers, that I can play it every time I like :)

Ironically the only game that has been constantly on my PC since I switched to Win 7 is HL2. Not because I like it, but I want to finish it before I can finally uninstall it ...

System Shock 2
Thief Gold
Thief II
Dungeon Crawl Stone Soup
Descent 1 + 2

Funnily enough, nothing new at all. I like some newer games, they just don't seem to have the staying power of these.....nostalgia kicking in I suppose...

Haha, I had Gothic 2 installed for a long time for the very same reason as SimonG.

I was close to the end but somehow lost my interest in game. Thought about finishing it for a long time.

But one drunken evening I said "Fuck this shit" and deleted it. And never finished it to this day. And I don't regret it a bit :D

Diablo+Diablo II

MAME and its ROMS

Several DOS (and DOSBox-tailored) games like Prince of Persia, GODS, Flashback, Mortal Kombat, Another World, Pinball Fantasies etc.