I think it would be easier to tell you which to avoid than which are good.
As long as you stay away from parts guns that show up at gun shows sometimes, you should be OK. By parts guns, I mean guns that have name brand upper and lower receivers, but the rest of the gun was assembled with whatever the cheapest nonbrand/surplus/factory second parts they could lay their hands on and put together by someone who may, or may not know what they're doing.
Another name to add to the good list: Armalite. I've been very, very happy with mine. Not easy to find in most shops for some reason.