Radway Green. NATO 5.56 made by the British.
The other alternative is the Guatemalan 5.56. There's also the 5.56 made in the United Arab Emirates.
While I am sure both of those are fine, I trust British ammo a little bit more for some reason
Finally, there is the XM855PD. That's made in the US, but it isn't factory seconds, they are factory thirds. It is too expensive to justify being thirds.
For new factory ammo, there's Winchester Q3131, which is 5.56 ammo. It is expensive. AE is next, it is .223 pressure, and it is expensive, but less than the Winchester. Rem UMC was the cheapest of the factory .223, but they just had a price increase.
For blasting ammo, Wolf is hard to beat. If your AR can eat it. My Colt has zero problems with it.
That's pretty much the breakdown. Your best bet is to go to ar15.com, go to the AR15 forum, then to the ammunition discussion area and click on the Ammunition prices sticky. Whatever is listed there, is pretty much what's available and for what prices.