Re: upper what do you like??
Custom built by me for most of them. Now and again I'll get a good one if it is the right price and offers most of what I want. I just picked up an AAC .300BLK, it seems pretty nice. My LMT M4 was a 14.5 upper and I got their SOPMOD lower and just pinned 'em together for a rifle. Turned out nicer than the NIB Colt I had in the army.
For fun stuff, for my M4 and less accurate or medium range work, I like the good, decent quality stuff. I think LMT is the best "standard military grade" AR by which others should be measured (note "standard" --there are better and worse, I just feel LMT is best all around in most categories). For the high end, I build all those from parts I am comfortable using. The good ones pre-built run $3000 or more and STILL don't have all I want. For $1500, maybe a little more, I can build a better one that has everything I want and save enough money to sometimes scope the rifle.
For long range or accurate rifles for instance, I like cut rifle barrels, like Satern, etc. I like the JP Ent. "Tactical" bolt carrier (best made if you ask me). I use KAC rails when I use rails, usually the FF ones, but the M4 mil version on the LMT is better than half the other FF rails out there anyway. I also use the KAC rear flip sights too, they actually work pretty well for what they do, even out to 600m. I use the AAC Blackout on most rifles so I can use the SDN6 when it gets here. Needless to say, not many ready to go uppers are made to that spec. Lowers complicate it further for me, as I'm just as picky about the trigger and the stock assy. (yet prefer milspec small parts for the rest).
And yeah, I hear lots of good about Noveske, KAC and JP Ent. (of which I've used and can vouche for) and never heard bad of LaRue either. Some of the better ones cater to certain groups too. JP for instance, they cater a lot to 3gun (not to say their parts aren't good for other things) and Noveske caters to higher end military gear. Their barrel cut using a SAW blank, just wow. Those barrels are tough, probably too tough.
Yeah, I'm a bit of an AR snob, but I've had one or several since that first one in ('95?). It is my favorite platform by far, didn't intend on it, but it grew on me.
Conversely, there are a lot of shady places cashing in on the AR rush. Many companies getting cheap parts and having their names put on it and claiming to be high end. Grossly over-priced rifles that have "completely redesigned the Colt drawings and made patented changes that are secret measurments and design changes that optimizes the platform". They usually go deeper into technical jargon to ooh and aah the audience and to convey a sense of superior technology. However, the weapons look identical to just about any AR out there and in fact use all the usual parts. Hence, I only stick with well established makers with a good track record and/or those with government contracts of some kind, depending on the application.