I have 387 hours logged on Skyrim in Steam, but I'm pretty sure I have a bunch more than that for Civ IV - I just played most of that before I got it on Steam (I "only" have a little over 100 hours logged on it in Steam). But my most played is probably Final Fantasy IX. I'm fairly sure I have somewhere between 1000-2000 hours in that. A few other FF games probably beat at least Skyrim, as well, though I'm not sure exactly which ones.