1.
Quakeworld Team Fortress - The first mod to ever make it big, and undeniably one of the most amazing games to ever be created for free
2.
Fortress Forever - I am currently beta testing this yet unreleased Half-Life 2 mod, a follow up to QWTF, TFC, and the proper heir to the TF throne, it is balls to the wall fantastic...and FREE!
3.
Day Of The Tentacle/Sam And Max/Space Quest 4 - Collectively the three most hilarious adventure games ever created, I still play them all every year or two on an old Windows 98 rig I have built specifically for pre-1996 games.
4.
Battlefield 1942/Vietnam - The last two installments have tried but have yet to match the awesomeness of the original, or the jungle fun of it's sequel.
5.
Counter-Strike/Counter-Strike:Source - I am insanely good at twitch gaming, so these are both tops for me. BOOM! HEADSHOT!
Honourable Mentions:
TFC - Team Fortress Classic, the Half-Life remake that was slightly slower and about 80% as fun as the original
C&C Red Alert - Still the best RTS ever made
Police Quest 4: Open Season - Insanely inventive and in depth adventure game
Civilization 4 - Best turn-based strat ever made, most balanced and
fun in the Civilization series
Duke Nukem 3D - Epic's masterpiece, turned the world of gaming on it's head with violence, sex, and loads of toungue-in-cheek humour