In terms of ratio of gaming dollars spent:enjoyment, it’s hard for me to beat Audiosurf, but that was also only $10.
For games I paid a normal computer-game price for: GalCiv 2, X-Plane, ArmA and Homeworld. Also let me, like, dozenth Civilization 2, which was the reason I did not sleep at all one junior high school summer.
I think Baldur’s Gate 2 beats everything else for me, right now. I have literally replayed that game more times than I can think of; the player-created free content helps a lot there. I replay the game at least every month or two, in total or in parts.
Planescape: Torment. I got it for $9.99 in the bargain bin. Never spent more time or mental energy on a game for that amount before or since.
I have played this game several times, and a ton of the games on this thread. Still, DK beats everything. It’s my version of a first person shooter in some ways, I play it for stress relief.
Any Spiderweb Software game, but especially Geneforge II, which I have played all the way through at least a dozen times. It’s not a short game, either, so that’s a significant time investment. Best $28 I ever spent.
Dollar per hour, it’d be Civ II for the PC, followed up by Pirates! for the Commodore 64.
Now that I think about it, GameTap is a pretty awesome deal, especially if you don’t mind playing older games. There are about a zillion games there, so obviously there are lots of bad ones, but there are some uber-classics, too - Deus Ex and the Fallouts, for example.
Planescape Torment.
World of Warcraft. :eek:
World of Warcraft, no contest. As has been pointed out, even paying for 3 games plus $15 a month, it has given me countless hours of fun and reduced my interest in buying other games.
Half-Life takes the cake for me. I played the single player campaign about a dozen times, and I spent countless hours on CS, DoD and other mods.
In terms of hours played in relation of cost: Starcraft, Super Smash Brothers Melee, Diablo 2, Unreal Tournament 99, and Orange Box (TF2). Maybe Soul Calibur 2 as well. Countless hours lost to those games, thanks to the multiplayer component and having friends that really liked 'em.
Other great ones that are single-player only, so they don’t stack up in the time category, but are great games I play over and over again: Final Fantasy VII, Deus Ex, Doom (which is multiplayer, but I only play it for the SP).
Either WoW or Rock Band downloads. Seeing my 56 year-old father sing “Margaritaville” is priceless.
StarCraft. Halflife. Civs I, II, III and IV, Europa Universalis II and III. But probably the biggest is SMAC. “Please, don’t go. The drones need you. They look up to you…”
Had to be the $10 I had on a 27-1 longshot some years ago. The longshot won, and I collected about $280 at the mutuel window.
Oh … you don’t mean that kind of gaming. 
Seriously, I’d have to say Microsoft’s Pinball Arcade, which lets you play a number of old Gottlieb pinball machines. I’ve never been a fan of big and complicated games that require a lot of concentration and/or that are played over long periods of time, so something that lets me play for as little as a few minutes then return to work without wondering where I left off is perfect for me. And I also like pinball, so this game (which replicates some of my full-size real-life favorites) is fine by me. I’ve had this pinball game for about ten years, and have ported it to about three different machines as I’ve got new computers and retired the old ones. Haven’t got tired of it yet.
A $20 shareware game for the Mac called Escape Velocity, and its sequels, EV Override and EV Nova. Completely open-ended space trading/combat/exploration games with a plug-in architecture for unlimited expandability. Kept me busy for years.
Diablo II gave me a good long ride.
World of Warcraft
WoW worked for me for a long time. Also:
Wing Commander Privateer
Lords of the Realm II
The Quest for Glory series
Must have played hundreds, if not thousands, of hours with the original Unreal Tournament (main draw: hilarious custom skins and voicepacks), but even those hours have been easily exceeded by the Heroes of Might and Magic games, II-V (IV not so much). Civ II-IV probably would be third on my list (tho I’ve forsworn it for about a year now), followed by the various iterations of Nascar Racing (and GPL; same publisher). I dislike games which have a “story”, and you don’t have any motivation to keep playing it once you reach the end of the story (unless you can mod new stories).
Planescape: Torment
Chrono Cross
Okami
Psychonauts
I’ve been playing the hell out of WoW - for less time than EQ, but it’s also caused me less pain. And the Rock Band downloads are a good choice.
Oh, and Rock Band has introduced me to music I haven’t heard before and have really enjoyed. On opposite ends of the spectrum, after singing [url=http://en.wikipedia.org/wiki/Lacuna_Coil I have a fair amount of their music on my iPod, and I really liked the Dead’s song “Alabama Getaway” and am open to checking out more of their music.
I’ll add my name to WoW. That game covers a lot of play time for me.
Second on the list is Resident Evil 4. I’ve played every incarnation of it repeatedly.
Third is an online game trading service called Goozex. I’ve unloaded and received a lot of games off this site.