MR works best for me. There's a little "break-in" period when converting from FR to MR, but once you've got the hang of it, it makes it so much easier...
MR and Fr are my favorites. 4WD should stay in the dirt but the best 4wd has to me mid mounted 4wd (e.g. ford RS200). Oh yeah RR is crazy (Ruf Yellow Bird On a rally track in GT2 anyone)
I like the MR 'cause I can not do spins @ high speed wit high horses. & drifting is better. Then 4 WD (a little restrictive on the horses). Then FR & least of all FF & RR.