One of the great things about the AR-15 system is the ability to have multiple uppers for just one lower. It can save money and if it is important to you, weight and space depending on your needs.
JP Rifles complete 16" upper with 15.5" handguard and Troy BUIS
For the one lower, you can have uppers that cover several calibers and provide great ballistics for various ranges. While personally I prefer to have a lower for every upper I own, I do love the ability to buy an upper, zero it, test it and then either select the best lower for it or build a lower to suit it. As an impatient guy, I can almost feel like I got a new rifle without the hassle of an FFL transfer.
What upper is best for you is very dependent on your needs and budget. There are many great options out there but the choices can be overwhelming. If I had to offer just one upper I would suggest a Bravo Company 16in with YHM rail. To me this upper offers the best non-sale value in terms of price versus performance. Just note that you will need to buy a bolt carrier group (BCG) and charging handle also, which will run you about $130 extra at the low end of cost. Also consider LMT's complete uppers for this category.
If you want the cheapest acceptable upper you can get, it is hard to go past DPMS. With an account at Brownells or a good deal in a store, you can easily get an upper with bolt carrier group for under $500. But for that price it is unlikely you will have a rail system and free floating barrel unless you get something heavily discounted. Additionally, at that price you are probably better off just getting the cheapest complete rifle you can. You will certainly see brand new uppers for $300 or just over - but they don't include a BCG - so add $120+ and possibly shipping from 2 places and you are getting close to $500 again, generally with a worse barrel. Also consider any upper sold at Bravo Company, or by YHM. Palmetto State Armory also offers some very competitively priced uppers, but I have not tested any and there are some reports of out of spec parts, though they will accept a return should you have an issue. Surplus ammo and arms can have some great deals on WOA barreled uppers with a free float rail also
You can't beat JP Rifles for 14.5" and longer. Their JP-15 series is cheaper than many other high end brands, only you get a QPQ BCG, an adjustable gas system (tuned by them), an improved bolt and for a little extra you can get the heatsink. A solid competitor is Noveske, who also have some great SBR options.
I am not a gas piston guy, but LWRCI also does awesome work. I talk a little more about them on my complete rifle page.