Doom 3 or Quake 4?
How about
Doom 4?
But, seriously.
When Doom 3 came out for Windows, I just had to get it. I bought it and could play all settings max with Dolby 5.1 sound... one word: WOW!! So scary!
Must admit that after a couple of hours it did become a bit predictable... i.e. enter room, admire the grfx, room becomes dark, <Quick Save>, be shocked by awesome monster, kill monster, next room... is dark... sound of falling bolts behind you... get startled by next awesome monster.. forget Quick Save, and reload from last Quick Save.. etc.

And because it was so gr8, when it came out for the Mac, I immediately bought a copy Doom 3 for Mac too! I got the GeForce 6800 Ultra in my then Dual 1.8 GHz G5, and wanted to find out how gr8 that card was...!
I have saved all of my save game files, and now and then I fire up Doom 3 and reload one of my final saved games, just to look at it!
I also got Quake 4 for the PC, and now I have a Mac Pro, I can also install it under Mac OS X, as the serial for Mac and PC both work

But, in fairness....I hardly ever played it. Yeah it looks cool but because I have played Doom 3, Quake 4 doesn't seem to cut it.
I am sure if I had never seen Doom 3, I would probably thought Quake 4 was awesome... but not now.
Can't wait for Doom 4.