I've bought a ton of AR platform rifles over the years...I've also sold a few. Here's a few comments....
1) You want something with a 1:7 twist. A lot of cheaper consumer rifles have a 1:9 twist (including Bushmaster).
2) You want M4 feed ramps, again, something the cheaper consumer rifles skimp on.
3) You want a full auto bolt carrier group that has been high pressure tested (HPT) and mag particle inspected (MPT) with a staked gas key.
Then, you need to consider the rifles purpose/ your "mission".
Colt - some Colt rifles use large pin holes so receivers (upper and lower) are not always compatible with just about every other manufacturer out there.
If you want high end - check out what Rainier Arms sells - they don't sell junk (Noveske, Daniel Defense, LMT, etc). Their prices are a bit high, but they specialize in high end stuff (yeah, if you look around, you'll find the same rifles for less) =>
Rainier Arms - Pursue Your Passion
If you're looking for a workhorse type rifle, check out Bravo Company. They also have higher end stuff and their own brand is very good. I have a pinned 16 inch pencil weight barrel (they pin the flash hider to make NFA requirements around minimum length) upper with their bolt carrier group and their lower. I've shot the heck out of that rifle and it just keeps ticking. =>
BravoCompanyUSA.com
If you want a good consumer level rifle then as you say, Colt, Ruger, S&W, Springfield (Saint), etc.
If you are bargain hunting, check out Palmetto State Armory =>
Ammunition, AR15 Parts, Magazines, Bulk Ammo - Palmetto State Armory I have bought a TON of uppers and parts from them (be sure to pay attention to what you're buying especially with the points I mention above). They have basically three level of rifles - junk, middle of the road, and good to go. Here's how it works on their uppers:
CHF - these are barrels made by FN and stamped as such. It's "machine gun steel" and rated to be the most durable. I don't do mag dumps, but I own one of these in an 18" midlength gas rifle with a 1-6x 24 Trijicon and it's an awesome rifle.
Premium - Decent rifle with a few upgrades. I believe these are also FN barrels but not marked as "CHF" or "machine gun steel". I've got a few of these in various chambers - 5.56/.223, .223 Wylde (1:8 Twist), 7.62x39, and .300 Blackout.
Freedom - Budget grade. Still milspec, still decent shooters, but not as good of a quality.
PTAC - Super budget grade....similar to Delton. I've never bought a PTAC rifle or upper and can't comment.