Oh boy, can of worms. The ones to buy, Yamaha, Honda, Kawasaki and Suzuki. CF Moto is a new kid on the block however so far they seen to do well in reliability. Of the above 4+1, you may have to travel a bit to find a dealer but don't rule out the one you want because of that. The above models generally cost a bit more and fall under the caveat of "you get your moneys worth", rather than I bought the cheaper one but now I'm in the repair shop more than I want to be. Personal choice, Yamaha 450 or 700 Grizzly or Kodiak. More than strong enough and gentle on the gas. I've been around Yamaha models that had over 10K kilometers with just the basic maintenance, oil, tires, battery and maybe a lightbulb or two. Zero transmission problems, Zero belt problems, without a doubt Yamaha has the best drive train out there. Honda has grown up and improved on their machines, still lacking in ground clearance a bit.
As for the ones to avoid, in no particular order Polaris, CanAm/BRP and Arctic Cat, (now Textron). I've seen too many Polaris ones first time out with problems, electrical gremlins, drive train gremlins, belts getting soaked and having to be towed back. Not starting in the cold. Can/Am clean out of the showroom, transmission problems related to electronics. Leaky oil and rad pumps. Arctic Cat, leave them in the litter box.
There is another brand out now, Argo makes an ATV, the jury isn't saying much but rumblings I've heard, their ARGO 6x6 and 8x8 is what they are best at.
Undoubtedly there will will be some that say their Polaris has never failed or their Canned Ham is bullet proof, It's like trying to write a story at -30 with a ball point pen in the snowbank. You pay now or you cry and pay for repairs and rebuilds.