The best zero turn mower for the money is the Ferris Z3x 61" however they are expensive mowers.
The Wright ZK is also a great Zero turn mower.
John Deere Zero turn mowers are also great as well as the Ego Power+ 52" Z6 Zero Turn riding mower which has a 52 inch mower deck and also expandable battery system.
Another great zero turn mower for the money is the Cub Cadet Ultima ZT1 which combines raw power with extreme comfort.
The best riding lawn mower for hills is the cub cadet ZTS2 series which is rated for up to 20 degree slopes.
I use the cub cadet ZTS2 series at work in lawn care and use it on 25 degree slopes and it feels solid and does a great job.
It's not mine but my bosses mower but I would highly recommend it for hills and steep slopes.
Other best riding lawn mowers for hills and steep slopes are the Venntrac and Steiner mowers.
However these are more expensive lawn mowers and most newer lawn and garden tractors do very well on steep slopes but you also have to be careful.
The Kubota lawn tractor or X500 JD is also a good option for mowing steep hills.
Lawn tractors have more grip on hills and slopes than a zero turn mower.
Lawn tractors are always a better choice over zero turn mowers when you have a lawn that has lots of hills and slopes.
You can keep the front wheels under control and maintain grip while driving up a hill or slope with a lawn tractor and you can't do that very good with a zero turn.
Zero turn mowers are best when you have flat lawns and not many or any slopes or hills to mow.