Your list is incomplete, starting with not listing Scag or Ferris, and also note that ExMark and Toro are the same thing. That might help you sort this poll out easier and get a lot less "other" answers. My suggestion is ExMark/ Toro. We've owned 5 of them, still currently with 4. Sold the oldest one we had (just weren't using it much anymore). It was 10 years old, with 2000 hours, 60" cut, 28 HP Kohler EFI. Bought new for $8000, sold used for $2700.
The only time you can justify having a diesel is if you are mowing for hours straight, say 3+ hours per day on the same property/piece of land. Best for Municipalities and Large acerage account service companys. Can't go wrong with Kaw or Kohler. All of our units are EFI, I feel it's worth the extra couple bucks.