This is actually difficult to limit to 10 but I would say:
1: Dominions 3
2: Space Empires 4 gold
3: BF:1942
4: Half-Life series
5: Star Control 2
6: Archon 1 and 2
7: Master of Magic
8: Master of Orion (yes, the original
)
9: Worms World Party/Armageddon
10: Civilization series
Some other honorable mentions:
Spy vs. Spy series
Jumpman
Bruce Lee
Caveman Ugh-Lympics
Movie Monster game
Ultima series
City of Heroes/Villains
Morrowind
Ravenshield
Team Fortress 2
and many more. Some of these are pretty old, but every once and awhile I dust off one of my old consoles or my Commodore 64 and engage in some nostalgia.