For the AR15, the 6.5 Grendel will make you almost forget that you tried shooting distance before with .223, and it is the only 6.5mm cartridge specifically meant to work in the AR15 gas system when looking at factory ammo and brass options. You do not need a heavy, long barrel to realize the long range or superb accuracy of the 6.5 Grendel, my 17.6" Lilja Grendel or 16" AA Grendel being easy examples. Both are ELR capable with factory ammo. Just plug in the distance, atmospherics, dial up, make your wind call, and let fly, then watch in amazement as the bullet hits.
For the AR10, I like .260 Rem and 6.5 Creedmoor. The new Winchester factory load in the 6.5 Creedmoor has very tough brass compared to the Hornady. Barrel life will be less with these two cartridges, especially if you push them and torch your throat early on, but they slam into the steel with authority at 1000 yds...DANK!
For wildcats, any of the 6mm variants of the 3 above parent cases, as well as the 6.5x47 Lapua necked down to 6mm.