It really depends on what you are going to be doing with it and where you live.
I have a little air cooled 250cc 2 wheel drive Suzuki Ozark and am really happy with it. I live where there is rarely snow and I already have a 45hp Kubota tractor that I use for discing, planting, etc. so I don't really need a big work horse of an ATV.
I was also lucky to find a couple of barely used ATVs owned by an older individual and got a really good deal on them. I bought a 2004 and 2005 Suzuki 250cc that were both barely used(1 is a sport model and 1 is the Ozark) and a nice little trailer for $6,250 which is less than what one of the larger 4x4 models would go for.
The important thing is figuring out your budget andwhat you are going to use it for and buying the appropriate machine for you.
FWIW, Nathan