You ask a tough question. For everyone who suggests a good bike, you will probably have one guy who disagrees. The fact is , there is no one great bike. They are all good, some are better though. Usually price dictates that, but not always. I hope I'm not confusing you.
Since you're buying used, there are a few things that "may" help you. Look for low mileage bikes, look at the condition of the bike. How old was the rider, is the bike stock? Usually buying from an older gentleman or a lady could mean it wasn't driven hard or abused. Like was said before, if the bike was modded, it probably had a hard life....run away from those! I like asking the seller why they are selling it and if they say , they want something bigger, it makes me think they worked the bike hard. I like hearing "no time to drive it" or "just had kids and need the money" etc..... The best bike to buy is when the owner doesn't really want to sell.
Get as much advice as you can and make the purchase. Good luck to you!

 

What state are you from?

 

I'm in the western part of Colorado. I've always been interested in the Kawasaki Brute forces though I've never heard up or down about the reliability of them as well as I haven't had much luck with Kawasaki's in the past. I've had very good luck with Yamaha's so I'll probably try to stay with a Yami. (Though all of this is with snowmobiles.