Sure, ask an easy question!
But seriously, its really hard to say. I've got a near compulsive tendency towards strategy games and a passion for single player computer RPGs. And this isn't even considering console gaming from the past ? where I probably sunk another great chunk of time.
I suppose though, if I had to say where the bulk of my time were invested...
Alpha Centauri/Alien Crossfire almost definitely takes 1st place. I love it, especially the ability to shape the terrain ? something devs might want to come back to! Jesus, set a game in space and make terraforming a meaningless numbers game? BORING! Give me the ability to dig the suez for my empire!
After that I guess there'd have to be Company of Heroes Gold/TOV. I played mostly gold, but I add TOV on the grounds that it was such a meaningless expansion given its impact on my play style its hardly worth mentioning or leaving out for that matter. I'm not knocking it, it's my second MOST played game, I'm just saying.
I guess after that would be MOO2. I invested a good chunk of time in that game as well. Although I'm not sure if I should say MOO2 or Morrowind ? both seem to have had about as much play. Knowing me though, I'm guessing Masters of Orion 2.
Huh... All right, guess I'm a bit of an RTS TBS fan-boy. Funny because I love some of em and loath the rest ? all hammer no manoeuvre.