Kubota make an excellent ZTR especially if you want a diesel and Kubota content of the mowers is high (hydros, lift, engines). All the commercial machines are good and will provide a long life span if given normal maintenance.
It depends on your situation - residential or commercial, rough or smooth lawn, amount of obstacles/trees/shrubs to deal with etc.
I ended up with a Ferris IS2000 52" 25HP Kawasaki, full suspension as our ground is rough and it rides really nice. The other factor was cost, as I got an 09 leftover $7K vs $10-12K for the Kubota diesel - I dont put more than 40 hrs a year on the machine, so 20 years or 800 hrs the ROI for $3-4K more wasnt there for me. Now if you run it 40 hrs a week, thats an entrirely different equation for a diesel as you will save a fair amount on fuel alone it will jsuttify the cost difference.
There are lots of choices out there, Scag, Exmark, and more..
Good luck in your search,