Despite my tendency to not answer questions like this because of the HUGE varity.... :) I will say that my favorite would definitely be Appendix. I love WBS TO DEATH and that's all I own (other than the occasional TB training project and riding everyone's horse that I can :) ) but health wise and temperment and potential, I like the Appendix TB. I've never met a rude, or problem one, and *I do admit* that it depends SOLELY on getting the right mix, but I just love that breed.
Off track... but I once had one from a kill pen that I took home and next tihing you know, he's jumping the 6' car in front of him and took off for home. (However we were trailering home and stopped at a show to show off to my friend that owns the barn. So, he made it into that barn and took the nearest open stall :) )