Richard Burns Rally is the best sim rally game for PC to date. It's horrendously difficult, and has the steepest learning curve, but once you start to make those stages the feel (you must have a wheel with it) is brilliant.
Also, lokiss is correct on Colin McRae 04 being one of the better ones. I'm currently playing DiRT 2, and although it's a great game for many reasons, I find the RAID and Baja races etc a total chore. The trucks and buggies are awful to drive and it's not a lot of fun. I yearn for a non-Americanised rally game from Codies again, but I doubt it'll ever surface.