Kohan: Ahriman's Gift (with the community mod Ahriman's Betrayal) is my all time favorite multiplayer game. My friends and I get together every Sunday night to play.
Civilization II I never played the original, but 2 cost me many sleepless nights. With 2, I must mention Alpha Centauri and Civ 3... I've been disturbed too many times by my alarm clock while I was deciding whether to nerve staple my drones or not. I have given orders to shoot all Hive and Spartans on sight! Death to Yang! Long live the University!
Lords of the Realm II Great fun. Sure it was unrealistic, but it was fun!
Knights of the Old Republic I was totally sucked into the story... and I'm not a Sci-Fi person. Was I the only one surprised at the plot twist? You could have pushed me over with a feather when... well, I won't spoil it for you.
Warlords The original turn based fantasy strategy, afaik.
Lemmings Dig there... yes! Wait, noooo! Not there! Quick build stairs! Gaaaaaaahhhh!!!!!
Max Payne Gritty. Fun. Bullet time was too cool.
Diablo II Did not get around to the original, but 2 was excellent.
Grand Theft Auto: Vice City I've often wondered if this was the best or worst game I've ever played. Pros: a bunch of radio stations, tons of vehicles to drive, constant pressure by the cops and other npcs, lots of ways to cause mayhem, getting the penthouse suite with a helipad, Ray Liotta, Phillip Michael Thomas, Lee Majors, Tom Sizemore, Dennis Hopper, Burt Reynolds, Gary Busey, Luis Guzman, Debby Harry, and you get to punish scumbags Cons: you murder people, you are a cop killer, you pick up prostitutes, and you are a scumbag
Shogun: Total War, Medieval: Total War, Rome: Total War I just love playing the campaign maps.
City of Heroes The first six months were bliss...