M7 or king cat depending on what your gonna want to do with it. The M7 is the better boondocker and probably a better ride on the trails, I don't know that for a fact, though. I just know that it can't be worse on the trails than a king. If you're just wanting a pull the rope and go sled that can throw up a decent mark (for a stock sled) on the hill. then the king cat is for you. Both take well to mods.
If you're looking at the other brands, that kind of money will get you a nice ZX summit, or early rev. Nothing wrong with either of those sleds, my brother in laws first sled was a 800 ZX and he still rides it and enjoys the heck out of it. Probably want to make sure the carb boots are in good shape they've been known to delaminate.
You could get a polaris edge RMK 700-800 for that money. Both are strong performers with the 700 getting the nod in reliability. The 800 liberty is a great engine with it's main weak point being the crank. I would definately look to see if any crank work has been done on any potential 800 polaris purchase. I've seen at least 3 800's come out of the backcountry on the end of a rope because of broken cranks, and I don't ride with many polaris'.
If you're looking at a yamaha, you would probably be looking at an 2003-2005 RX-1 for the money you want to spend. I haven't been around many of them, so I can't speak intelligently to their pros and cons of the chassis. The motor is rock solid, but the weight is a huge factor to consider if you are looking at one of these. A mountain viper or mountain max would also come in around this price, but I really don't know much about them aside from they sound really cool with pipes and seem to have a good reputation for reliability.
EDIT: Man, work is slow today....