Hard to say which is the most.
RPG's:
Deus Ex and Baldurs Gate 2. Probably thousands of hours total on BG2. Mostly because about 3/4 of the game is optional and can be done several different ways. Also because every so often I find something that makes me go back. Like the Tweak Pack or that patch that lets me choose any resolution, and it actually works.
Probably several hundred hours on Deus Ex. Every level can be done at least two completely different ways.
I've likely played Morrowind for thousands of hours and Oblivion for hundreds. Oblivion might be the better game but Morrowind was ground breaking for me whereas Oblivion was just more of the same but with better physics and fast travel. Actually, fast travel is probably why I spent much less time playing it. Ditto Fallout 3.
RTS:
NOT Starcraft but I did play that a lot. Overall is probably Age of Mythology. I dont know why. Its just so much fun. Probably thousands of hours on that. Ditto Dawn of War, and all its expansions. Several thousand hours total.
Act of War. Again, not special other than its extremely well made and loads of fun. Several hundred hours.
TA, not so much. TA Kingdoms, tons. Probably thousands of hours. Way back when it was popular you could find loads of custom units all over the web. I probably spent just as much time downloading and tweaking custom content as playing the actual game. The only downside was the online match maker stunk and the AI was not adjustable, so when I started I got hammered, and as I got better the game was weak. Got tired eventually.
TBS:
Mostly 4X space games like Master of Orion. But overall most time is probably Heroes 3 Complete. Buttloads of campaigns each with tons of missions, and some missions can go for days. Thousands of hours altogether I'm sure. Disciples 2 is a good one, but I dont think I put nearly as many hours into it.
Racing:
TDU. It takes a long time to get everything and its also a shitload of fun just to drive around and do nothing. Much safer and cheaper than real life. If the online part didnt suck ass I would be spending all my time on it. They wanted it to be the first racing MMO and it never worked. TDU 2 comes out soon and I'm sure I will piss away a thousand hours on it. Assuming the gameplay doesnt stink.
Sports:
None.
Platformers:
Mostly the consoles, but on PC I havent spent too much time on them. Braid mostly, and some Jets N Guns.
Overall number one time sucker has to be Dawn of War. I've done the original and Winter Assault campaigns multiple times, as well as the lame ass campaigns found in Soul Storm and Dark Crusade. And theres no other game I'd rather jump into for skirmish. If the multiplayer worked as well as Battle Net, that game would have been MUCH more popular than Starcraft, and SC2 would have had some real competition to beat.
