If we're talking "out of the box", then GTR2 would get my nod, followed by GTL, then rFactor, then GPL.
However, because of the great mods and tracks, rFactor gets more driving time from me right now then all my other racing sims combined for the past several years. Yeah, there are some bad mods, but I tend to stick to just a few. The Formula BMWs, Lupos, Meganes, Super Clios, FSONE 2006, and Demons/Protons have taken a lot of my time.
After rF, I'd say GTR2 w/extra tracks and GPL 2004 w/extras are running tied for 2nd. GTL is my only one still in "ootb" condition, although I might have a couple of extra tracks for it loaded. The main problem with GTL is I got rF not long after, so I've not had nearly as much time in it as I'd like.
Doesn't help that I fly and play tac FPS and RTS as well as race!