Mine would have to go:
Starfleet Command: Orion Pirates
Civilization IV: Beyond The Sword
X-Wing vs TIE Fighter
World of Warcraft (spend way too much time here, but meh...)
Command and Conquer: Tiberium Dawn
Command and Conquer: Tiberium Wars
Command and Conquer: Red Alert 2
The gameplay on Tiberium Sun was a bit lacking especially for MP, Tiberium dawn's missions are just classic, and its good too, since multiplayer was added as an afterthought for that game, and it shows. Red Alert was better for MP, but not by much, and the campaigns weren't as good as TDs.
I doubt there could be a game to rank up above Orion Pirates, even though I spend more time on WoW, if a OP server were to come up, I'd drop playing time there to almost nothing.