Upper

I don't have any experience with AR's, or at least not enough to matter. My question for you, my discerning readers, is what brands/configs of upper receiver you have had luck with and would recommend to me, the discerning buyer that I am.

I'm half tempted to turn this AR build into a range queen kinda thing - buy a 6.8SPC upper, or a 6.5 Grendel or .50 Beowulf, something like that. Then I think about it as a housegun and flip back to 5.56 NATO.

This is why modular weapons aren't good for the compulsive buyers among us, myself included. I can already see the laundry list of uppers that will inhabit my safe sometime in the near future....

So, any ideas?