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 ...

Dude, you're almost at the end. One evening and you're done. Add one more evening for E1 and E2 each and that's it. And it does get better. (Well, E1 is kind of shit, but at least it's very short.)

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...

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.