For tractors - #1 Simplicity if you get one of the models with the Briggs and Stratton Vangaurd motors, close #2 John Deere (not the box store ones, the real Deeres with Kawasaki motors) for tractors. Stay away from Kohler motors, every one I've had has been junk. Kubotas are supposed to be great, especially if you want diesel instead of gas but very very expensive for a lawn mower. Cub Cadets look tempting, lots of bang for the buck there but MTD scares me. I had a very expensive MTD built Cub Cadet in the 80's, awful piece of junk, maybe better now, I just don't know.
For zero turns - I'm not sure. I know the John Deere and Skag ZTRs are excellent, my brother-in-law is a landscaper and uses and abuses both with no problems but there may be other good ones too, like maybe the pro level Toro ZTRs (not the box store ones).
Good luck!