I really love the Half life series, I've saw it when I was little, and remember the game and started the Half life series to Half life 2 then the episodes. Can't wait for episode 3!, and smod redux is actually my favorite mod, TF2 is my favorite Mulitplayer game, while DOD:S is a game I can have fun with its realistic gun sounds.
Singleplayer: Crysis/Warhead I replay these a lot, but the Half Life series is pretty much up there with Crysis but I don't as nearly replay Half Life campagins as much as Crysis and Warhead.
Multiplayer: Team Fortress 2
Future singleplayer and multiplayer would probably be Left 4 Dead.
All time favorite series is the metal gear and metal gear solid games. But I gotta say my favorite one is definitely the original Metal Gear followed very closely by Metal Gear Solid 3.
Hard... hard... I play so many different games, ranging from WoW and Warhammer: Age of Reckoning to Rome Total War, Medieval II: Total War, Battle for Middle-Earth (I+II) to DoD:S and INS
I guess I'd say WAR, BFME II and DoD:S. If I'd have to pick one top-favourite, DoD:S it is
CS beta 5.2, but any CS as well
Tekken 3 - PS
Goldeneye - N64
Super Mario 3 - NES
Tribes - PC (wish I had a good PC back then it would've been nuts)
Halo - XBOX
Diablo 2 - PC
Legend of Zelda - NES
Ninja Gaiden - XBOX (Even though you almost have to be a ninja to play it)
Currently I'm addicted to DoD. I've finally figured out how to appropriately use most the of the guns so that makes a HUGE diff. Wewt I don't just have to use the M1 now!
For time spent It would be City of Heroes/Villians but somehow I never felt accomplished playing that one.