The Kawasaki Mule is a very proven model and can be had in a diesel. They seem like a very good choice for dependability, and a good choice as a used model. I see a lot of them being used by cities and counties.
If you want a new one for hard work and a long life the Kubota RTV seems like a good choice. I see a lot of them being used in the Central Valley. They are not for fun, high speed, trail use because they have a very heavy duty rear suspension, but probably as good or better quality than a Kawi, if that's possible.
I'd steer clear of a Polaris for real work or longevity, especially in a used one. They are constantly upgrading them to fix stuff, it seems. Not refined before they sell them. They are famous for belt problems. Polaris, unlike Kawi or Kubota, seems to go ahead and sell their stuff, then let the public find the weaknesses and fix them for the next years model. But if you want high speed fun in rough terrain, the Polaris Razr looks like a blast, and very powerful.