In terms of pure value for money, I'd say UT2004. There's such an unbelieveably huge pile of free extra content available for it that it's like buying 50 games for the price of one.

Well you have to give us a genre.

Pick an Infinity Engine game, any Infinity Engine game. :D

Currently it's Beyond Good and Evil for me, but ask me again and I will give a different answer.

(in before Licurg)

You might as well ask "what game do you like?"

I doubt the answers will tell you anything

Planescape: Torment is the best game ever no matter how much you pay for it.

You're welcome.

Fallout 1 or 2 (all around best)
NWN Diamond (longest, most bang for the buck)
Battle Realms (fast, fun, UNAPPRECIATED!!)

Deus Ex: GOTY
The Longest Journey
Freespace 2

The controls killed that game for me. Yes, I want to flip the Y-axis. No, of course I don't want to flip the X-axis as well, no game ever did that. Oh, that's all you can do, you say? Either flip both axis' or none of them? Well, I'll play something else, then.

System sho... oh wait baldur's gate 2

Quest for Glory 1-5 ($9.99 for full set)
Icewind Dale 1 & 2 ($9.99 each)
Baldurs Gate 1 & 2 ($9.99 each)
Giants Citizen Kabuto
Sacrifice
Prince of Persia 2008
Splinter Cell
Rainbow Six
Ghost Recon
Medal of Honor War Chest

Few of my favourites with that list. These are games I have gone back to over and over again and get as much enjoyment out of them each time I play.

It all depends on what genre you are actually looking for

That's too bad. It's one of the very few games (along with gems like Silent Hill 2) that I would characterize as sublime.

Neverwinter Nights.
While many games have better official campaigns, nothing offers the sheer volume of quality player made stuff (mods/haks/overrides/etc) of NWN.

The game itself might be, but the PC port sure as hell wasn't. And it annoys me a lot, because people generally only have nice things to say about it, apart from the horrible controls.