Buying a zero turn is like buying a tractor. First, set your budget on what you want to spend. Then the most important thing is going to be dealer support. When I started looking at them, I read all the posts that I could find here and over at Lawnsite. com Of the dozen or so top brands out there, only a few have what I wanted in dealer support. Some were sold at Ace Hardware, others from a Storage Unit lot and at the box stores. I wanted a dealer that could help me out with a problem and either have parts in his store, or able to get me the right part when I needed it. I wanted that dealer to be in a reasonable distance from me. While there might be better mowers out there then what I bought, I couldn't find one that fit my requirements better then Scag.
I can't possibly say that Scag is the best, but it is one of the top brands out there. I spent $8,000 for a 60 inch Wildcat that has a water cooled 26hp Kawasaki engine. It also has an oil cooler and is built like a tank. I have yet to find a hill that I can't go up, but my land isn't very steep. I do mow at angles that I would never mow with my tractor and finish mower. It just sticks to the ground and does an amazing job. I recently took off my blades to sharpen them for the first time, which was extremly fast and easy. Scag recomends greasing it every 100 hours. I have 20 on mine so far, so it's not much of a chore to keep it greased. I have solid front tires wich eliminates flats, but have had one flat on the rear tires.
I have two hydraulic pumps and drive motors for each rear tire. This is how commercial mowers do it. If you are looking at spending less, then some of the residential or estate level of mowers have one pump that runs both tires. Of the box store mowers that I looked at, the metal was very thin and a few were leaking fluids just sitting there. Nobody seemed to know anything about them, and I lost interest pretty quickly. I think that the more commercial the mower, the longer it will last and the happier you will be.
Good luck,
Eddie