OP8, and here’s why:
As you may or may not know, as you increase your OP level, you can choose whatever OP level you want to play whenever you start a game, so OP8 gives you the highest range. Also, you may use any-level gear at these levels, as in, you can bring OP8 leveled-gear to OP1 or OP3 leveled-gear to OP8, etc.
So here’s what you do: pick a middle ground. I use OP3, so let’s use that as an example. When you get to OP8, you’ll probably be using fairly on-level gear just to get through Digistruct Peak, so use that to go back to OP3 and farm your OP3 library. This will take some time, but at least the enemies will be a snap with your burly weapons. Or trade for them, whatever, just get your favorite weapons and shields at OP3. This will be your “library”.
Now when you play, select your desired difficulty by which OP level you bring this library to. Play OP0 with your OP3 stuff, and it’ll be fairly easy. Play OP8 with your OP3 stuff, and it’ll be harder than you thought OP8 was possible. You get a sliding scale of difficulty every time you start a game!
But! The reason for picking a middle ground is to give you some headroom to “buff” some of your gear. Think an e-tech dart pistol is too weak to use in UVHM? Add it to your OP3 library at OP8, and in the context of your OP3 library, this pistol will have damage on par with other pistols, making it viable again. Think the Commerce is pretty cool, but it’s just a little underwhelming? Add it to your OP3 library at, maybe, OP6. Tired of your nova shields tickling enemies when they proc? Add them to your OP3 library at OP8, and they should be fairly devastating. Like the glass-cannon mechanic of the Bee shield, but don’t like one-shotting everything? Add it to your OP3 library at OP0, and now it’s nerfed. (For example, I nerfed mine down to level 65 to get it to a comfortable level for myself).
So, when you pick an OP level to “park” your character at, pick one that gives you the most wiggle room for alternate difficulty selections and enough headroom to buff the weakest in-game gear you like to use to a level you prefer. In practice, playing right at OP3 with this library is perfect for me… I might only wander one level up or down, but the entire option is there. Maybe you’d like, say, Bandit e-tech assault rifles to be a few-shot death machine? That will probably need a full seven-level buff, so you’d want to park at OP0 (which leaves no room for a lower difficulty selection), but that’s how this works.
So, in answer to your last statement about missing Brawn Salvador… how much nerf do enemies need to make him as viable as you want? One? Two? Make it happen! Think sniping isn’t viable in the OP levels because you think Jakobs rifles should one-shot anything on screen without slag and this isn’t happening? Make it happen! Want Cloud Kill to one-shot enemies? Well, that still isn’t going to happen (the most I can get it to do is a total of like 1800K damage, but then everything else you do is way too strong), but you get the idea.