This is the answer for an ATV.
New or used I'd be looking for a Yamaha 700 Grizzly.
(assuming you meant 4-wheeler)
Some guys like Honda.
We move cows in some fairly rough country with ATV/UTV's and the Grizzly is a very good machine.
If speaking hard use and a UTV I'd get a Honda. We have a 900 Ranger and it's been fine but I don't bomb around too hard in that machine.