Whenever one wants to bring order to the world, but on the antihero side of things, I have found a couple of good picks for that:
Neferata is the first one of all. She has the ability to install Covens that raise diplomatic relations with anyone she wants. She even has a handmaiden that can do the same thing without much investment. Furthermore, she can vassalize anyone and can influence politics from afar with her web of power. For example, make the empire have a random war with someone close to it, join that war for diplomatic treaties.
Vlad is the second, for the skill he has that raises 40 diplomatic relations with imperial factions.
Dechala is the third, after she conquers much of Cathay, but she can do it endlessly, with everyone. She has a building in her pleasure palaces that adds up relations with everyone every time you build one.
I don't count ogres, because they already belong in the Ordertide in a neutral fashion.
Dark Elves follow, but not very closely. You have an ancillary called Envoy that is rare and random to get and it is not reliable. In some playthroughs I had like 6, and in some, not one at all. Constant wars with the high elves will strain relations with the empire and bretonnia until well into the mid and late game, but I made it once before.
Nagash can be diplomatic but only in the early game. As you expand you will pass the threshold slowly but surely and they will all massively dogpile you.