It matters the terrain you are riding, trails differ alot from place to place. My biggest concern is not buying more then you need weight wise because it steals power. The tougher the tire usually the more it weighs.
If you want the tougest tire this one is a good choice.
ITP Holeshot HD ATV Tire | ATV | Rocky Mountain ATV/MC
The trails I usually ride dont really require a 6 or 8 ply tires. I usually save money and weight and go with a 2-4 ply tire like these.
Maxxis Razr MX ATV Tire | ATV | Rocky Mountain ATV/MC
There is some more good info in the thread below