Oh man, Jezz Ball was a classic. I'd have to go with Tetris though. Or COUNTERSTRIKE. I logged so many hours on that sucker.
I think Mortal Kombat all version and Virtua Cops both games are very interesting if you are into arcade type games.... well Counterstrike good one to be played on LANs and networks....
RIght now, it has to be assassins creed... it's totally awesome... there are some awesome games out there right now tbh
I play: Mirror's Edge (So much innovative) Left 4 Dead (So addictive co-op) Quake 3 Arena (So classic and perfect) I can't wait for: Wheelman (Vin Diesel rulez) Gears of War 3 (that's just GoW) Resident Evil 5 (I love horror games)
I got the new Tomb Raider Underworld for Christmas. I'm still playing of the anniversary version so I had a quick look to see the improvements and new graphics. I have to say it looks fantastic. I also have to admit a liking to Mario Karts on the Wii.
I only play "Return to Castle Wolfenstein: Enemy Territory" at the moment (it's freeware by the way and was pretty cool.... like 4 years ago )