Best brand of tall boots?

Both Ariat and Mountain Horse are great. I have a pair of Ariats (I'm in love with them, but they're pretty new so I can't give a good review yet) and a good friend of mine has Mountain Horse ones and loves them. You'd probably be fine with either, but its safest to go to a tack store or big tack sale and try them on instead of buying online if you think they might be hard to fit.