Personally, I don't think it gets any better than an appendix. I had one as a kid and own one now and they are just wonderful. Easygoing and hardy as a QH, but athletic as a TB. Both of mine have had a more TB build, but I've ridden horses where it went the other way and they still felt
like a TB rather than a QH if that makes any sense. I think it's the alertness, they are always right with you or even a step ahead of you. In my experience they are quick and tough, but eager to please. They're also pretty smart, which has gotten me out of more than a few jams. My current Appendix is the most excitable one I've ever met, especially under saddle she can be more than a handful. However she always takes care of me when it counts and loves to be ridden. They can excel at just about anything you put them up to.
Of course, it mostly depends on the individual, but I have had nothing but good experiences with this cross. I could probably write a book on why they are perfect but I won't now :) good luck!