I'm gonna get hammered by the AR studs on this site, but while I would agree that Troy Industries and Yankee Hill probably lead the pack; I bought a cheap set of UTG flip ups! You know, I loc-tited (blue) a set on my DPMS and they serve the purpose. POI moves as intended and I hit the 9 or 10 ring at 50 yrds every time. They are co-witnessed to a bushnell red dot, and remain as B/U sights. Very happy with the set-up.
If you're a "3 Gun" Guy or you are an LEO and bought your own stuff, you may want to invest in something a little more expensive.