At this price point there are a boatload of brands available. Stick with the popular brand names and you can't go wrong. What it really comes down to is what features you want or don't want. Some folks like the original M16 configuration with the elevated front sight. I like (and own) a couple of that are flat across the receiver and gas block bit have Picatinny rails across the top and gas block. I have mounted a set of flip up iron sights and red dot sight so I can use either one. They also come in various colors, camouflage patterns and plain old black. I started with a simple DMPM Oracle, changed the buffer tube from commercial to military size, added a flat coil recoil spring and drop in 3 pound trigger. I replaced all the furniture with Magpul's OD green furniture and added a front grip. These rifle, the furniture change-out plus sights I mentioned all came in under $700. I've put over 1800 rounds thru this particular gun without and issues. My other AR is a DPMS that is factory stock and while I've not put the same number or rounds thru it, I've not had any issues with it. Other good brands include Ruger, Delton and whole host of others. It really comes down to what features you want and the quality of materials and workmanship.

Rugers Model # 8500 is wildly popular and incredible reliable.

ruger 556 is a nice rifle and you can find it under $500

DPMS makes some great rifles

I have a ATI and shoots amazing never had a issue. Paid $499

The ruger is a pretty good place to start, or you could buy parts and basic tools for that price and build one yourself to learn all the ins and outs.

Just stick with a mil spec for your first, You can always upgrade & build the rifle you want.

For that price range I'd look into - Savage patrol rifle , ruger, and dpms. A gun made by any of those should be with in your budget and will be a nice weapon