Well I have a list too, but I can't decide which one is my favorite, because I had so many good memories from all of those games and if I were younger and my friends want to play some of them, I will be happy to do it again
Anyway I wil try to list them from 1 to 5:
1. World of Warcraft: The Burning Crusade
2. Counter Strike 1.6
3. Mu Online 0.97d version
4. Lineage 2 Chronicle 4
5. Grand Theft Auto: Vice City
Those I think are the ones that I have most fun playing on a PC, but I love the old school ones that I'v played on my nintendo ripoff "terminator" like - Super Mario, Legend of Kage, TMNT 3, Bomberman, Chip 'n Dale Rescue Rangers 2 and the list goes on and on...