A little more info - size of property, slopes, obstacles, particular ztr models, price range - would be greatly appreciated.
In the absence of any of that, the brands you mention are all well-respected, and known for producing good stuff. However, as I had a bad experience with newer Cub (looked great on paper, ran great until warranty expired, can't keep pto belt on it for more than 1 hour despite dumping a bunch of time & money into it, dealer washed his hands of it & me), I'd avoid it like the plague.
Grasshopper makes some awesome equipment, but is not well represented in my area (more on that later).
I've always liked simplicity ever since I had an old B-210 Allis, but again the brand has lost territory near me. Ferris is the same machine, or darn close, but I have issues with my local dealer.
Gravely is in bed with Ariens (or is that the other way around?) and is well represented in my area by two great dealers. When I was last shopping, I was looking for a machine like the ZT HD/Max Zoom series. Unfortunately, at that time, they didn't exist. Guess I was ahead of my time..LOL
Thinking of my local dealers for the brands you mention, I'd be leaning heavily toward Gravely. In my area, I don't particularly care for the Cub, Ferris, Grasshopper or Snapper dealers (arrogant, insulting idiots), and the Simplicity and G-hopper dealers are both 45 miles away.
If I were looking to replace my DC Silver Eagle today, I would look long and hard at the ZT HD/Max Zoom, Husqvarna MZ, and DC Magnum series machines. In my area, all are well-represented by courteous, knowledgeable, professional dealers.
Magnum
HUSQVARNA MZ6128ZT - Zero turn mowers