It really depends on your situation. ZTRs for open areas and walk behinds for smaller areas that a ZTR might have trouble with.
From my experience with ZTRs...I prefer John Deere's. Where I used to work, we had 3 John Deere 997's 72" deck mowers (two were brand new to replace our Exmark Frontrunners).
The oldest 997 was approximately 3 years old and has like 850 hours on it. The other two brand new ones are approaching on 200.
We didn't abuse our mowers by any means, but we didn't baby them either. The turf we mowed could be pretty rough at times and none of them have been in the shop for anything...and I mean anything -- minus the oil change or flat tire.
But to be honest, I think your mid-mount Deere's, Exmark's and Grasshoppers' are quality mowers. As far as the walk behinds, I've never used one, but I hear lots of good things about Scags.