Best is relative to what you want to do with it. With as little information you provided in the OP, there is no way to be able to give you a really good answer.
While it has been noted that the 300 blackout does do well in + 8" barrels, that is not to say that many other weapons (calibers) do poorly either.
Just for the record, pistol calibers like 9mm and 45acp, 38/357, 40 S&W 10mm, etc also will do well from an even shorter (than 8") barrel and were actually designed to be fired from barrels shorter than 6 inches and when fired through longer barrels actually exceed the usual manufacturers velocity and "power rating" and increase in velocity and ft pounds, etc.
As for suppression, 4 of the 6 handgun calibers I mentioned above all have subsonic loads for them. Actually, the 38 Special, are all subsonic, even the +P, though I don't know any AR that are chambered in 38357, but they could be with a few design changes, but I digress. There are however 9mm, 40 S&W, 10mm and 45acp AR's out there.
AAC/Remington has spent years trying to push the 300 blackout (which they didn't design and mostly pirated anyway but is a whole different topic altogether), but in their marketing push, they only mentioned the few calibers that the blackout does perform slightly better than.
Do you remember anyone from AAC comparing the blackout to 10mm or even 45acp for close quarters tests? I don't. And with reason.
True that supersonic blackout does perform better at longer ranges than a handgun caliber, but if you're going to be building an sbr, I can only assume that it's for shorter ranges where an SBR would be the obvious better choice. Sniping targets at longer ranges simply dictate different weapons and choices.
So, what are your expectations and purpose/reason for considering building an SBR? What do you want to do with it and what do you want it to do?