Really like everything it depends on what you want to do with it. Farm/Ranch work, play, sport, trail?
I am a certified ATV instructor as well as motocrosser/harescramble rider, so if possible no matter how trivial you think it is take a course on saftey and riding, it helps!
I right now prefer Polaris, alot of people will bash them for the automatic drive, and I didn't think I'd like it, but it is great and the rumors of belt slippage are bunk. Polaris makes good "all around" machines for decent prices and ranges.
As far as worst there is alot of sorting to do there as guys will bash others for the heck of it!
If you want to spend alot of money and go fast go with Yamaha. I believe the new Grizzley is the biggest work mackine out there now and some say the Raptor is the quickest stock?
Some handy add-ons/features you might consider would be a winch, racks, hitch, plow, mower, really you can do just about anything you want with them or to them these days.
A good source of info is [url]www.atvconnection.com[/url] forums and site, also feel free to E-mail me if you need more info.