Treks are good, also Giant ATX, Norco, or if you have a little mechanical ability, Motobecane from Bikesdirect.com are decent for the money. The one thing you really want in a Mountain bike are disc brakes, if you can afford it get hydraulic brakes and a full air fork. 1x shifting is also nice, and I am a big proponent of dropper posts.
Still recommend Craigslist. I got a decent mtn Ghost bike for $500 there. You won't get nothing but garbage much lower than that. So if you do get cheap, just be ready to replace it in a year or 2 if you're hard on it. (Which mtn bike courses tend to be.)