I've just begun racing LfS a few days ago and I like all S1 and demo cars a played so far. The XRG was interesting, because there you can learn a lot about making good set-ups. Little changes have already big influence on the handling.
At time I try to master the XRT. New tracks, or tracks with extrem narrow corners like cadet I almost try with the XFG in order to get a clue how to drive there. Sometimes I also like to drive the LX4 and the UF1.