My preference is for Kawasaki engines. Because they mow so fast, I prefer the agility, less weight, and less scalping of a 60 inch vs. 72 inch (unless you are mowing many acres of wide open area with few trees/obstructions). I am VERY pleased with my workplace Exmark - built like a tank. Scag is the favorite of the lawn care companies in my locality. This indicates to me that they are the best "value" regarding quality/price/service, again, in our locale.
Pablo's statement troublesome. My son bought an Ariens (rebadged Gravely) last year from an excellent dealer. He had to stay within a budget range and purchased the "entry level" pro model. He compared many brands at his price level and the Ariens had the best components/value he could find (engine, hydraulic drive motors, tires, etc.). His is too new to make any reliability judgements, but he is pleased going into his second mowing season.
He got a new "last year's model" for under $5,000 delivered. They're at $6,000 now.
https://www.ariens.com/en-us/power-equipment/lawn-products/zero-turn-mowers/apex/apex-52-kawasaki