The 458 SOCOM and the 50 Beowulf are the "top dawgs" for AR big bore rifles. If you want a REALLY big bullet...those are the best two options. You can get complete rifles from several sources, including Rock River (458 only.) Alexander Arms has the .50 Beowulf and is the best place to buy. I prefer the Beowulf, but you'll have just as many people tell you the 458 is the superior round.
The Grendel is probably one of the best "potential" rifles out there right now just because it is such a flat shooting and accurate cartridge. Again, Alexander Arms is the way to go...and Sabre Defense as well.
Of course, don't rule out the other AR big boys, including the AR-10 chambered in 243, 260, 284, 308, 35 Rem, 358 Win, 300WSM, 7mmWSM, etc.