In X-Box (not 360) Halo 2 and lately Doom once in a while.
I did not like ANY of the Call of Duty games - I thought they were really, REALLY lame with the boundaries of the levels and especially with the "Driving" or "Boating" levels etc.
I saw the new COD on my nephews X-Box 360 and was not impressed at all. The opponents are really stupid! My opinion is that Halo 2 on the X-Box hit the pinnacle for a gaming experience.
PC - Nowadays all I play on the PC is Scrabble. I think the peak for PC gaming was Quake (the original game). After that (my opinion) action games really went downhill. Actually even before Quake. I REALLY liked Rise of the Triad and the original Doom, but Doom 2 was just so sprawling that you could actually get lost in a level (I killed EVERYTHING on one level of that game, normal difficulty, and was never able to find my way to the next level. I don't remember the level any more, but it was kind of annoying).
I didn't like ANY of the games on the Nintendo gamebox I bought, and I think I had about a half dozen of them.
I doubt that I will buy an X-Box 360. I suspected I might when the price plummeted after Blue Ray beat HD-DVD to death, but the price plummet didn't actually happen. I am amazed that more people aren;t going over to Playstation, since you also get a free Blue Ray DVD player in the process, but oh well.....lol