I've owned the same Deere commercial 757 Ztrak with a 60" 7-iron (7 gauge) deck since 2007. I have driven several others in that time from Exmark, Scag, Bad Boy, and even Ego. I have not driven a Kubota.
Of those I've used or tested, Exmark and Deere are the only two I would really consider buying, as someone with several acres wanting a commercial-grade mower for personal use. If I had tested Kubota, based on what others have said here, I guess I might include them in the list too.
Comparing Deere and Exmark, there are some generalities across their product range, the most dramatic to me being ride quality. The Deere's are well-known for being super-stiff chassis, almost uncomfortably so. I spoke with pro landscapers who have had both in their fleets, and repeatedly heard that they like the way the Exmarks ride better, but they like the way the Deere's cut better, and also commented on parts support being best with Deere.
Things may vary locally, but at least around here, 90% of landscaper trailers are carrying Deere or Exmark. That translates to a network of always-available parts, as it's not too often you can go more than a few days without a mower in spring, having parts in-stock at the dealer is always key. As with CUT's, I'd guess Kubota is probably on-par with these two there, as well.
I was not impressed by Scag, Bad Boy or Ego. All fine mowers, but not better than Deere or Exmark, and lacking the same level of parts availability and support. I didn't really give any of them serious consideration, preferring to just stick with my now-older Deere.