It's a toss up between RPG and smart FPS, with TBS coming behind a close second.
It's funny, because there are several RPGs and FPSs that I love to pieces but among TBS games the only one that I find really addictive is Civ (Alpha Centauri included). That's not to say I haven't enjoyed games like MOO, Stars!, Disciples or Jagged Alliance, but none of these took control of my life like Civ, System Shock 2 or Fallout 2.
There was once a time when I really enjoyed adventure games, but nowadays they just isn't true anymore. I loved Monkey Island for example, but now I think that if the game wasn't so funny I wouldn't touch it with a ten foot pole. It seems to me that the gameplay in those Lucasarts(or Sierra) adventures was really awful.