Manneroth is more prevalent in the lore than in the games themselves. Probably the most recognizable pit lord would be Brutallius who was essentially the Legions version of the Butcher. Boi was so hard core he had his hands cut off to be replaced with swords and was really into murder. More so than most demons.
There are a couple other notables pit lords as well scattered about with a good chunk being elites like the one that greets you upon entering Outland.
Azshara is the apex of manipulative, possibly even the biggest narcissist in all of existence: She named a city after herself (among other things), would make her underlings go out of their way to get even a glance from her, used absurd amounts of magic to make herself look pretty, was able to even convince demons to do her bidding, never took anyone seriously (she thought it was amusing Manneroth tried to kill her), and believed only Sargeras was worthy of being her husband. The list goes on and on with her.
Think all that is in the novels; War of the Ancients if I recall correctly.
Which going back to her not taking anyone seriously; good thing. Otherwise she would obliterate everyone with little to no effort with no chance of getting a hit in. It’s actually hinted at prior to fighting her in her raid. If a player were to use an emote on her she will afflict the player with some silly effect, if not just kill them outright. The woman has everything anyone could ever want yet wants more and more, no matter how insignificant of a bump in power, prestige, or wealth it is.
Prior to her mutation she was on par with Kil’jaeden and Archimonde in terms of power in lore. Afterwards if push came to shove she could have potentially roughed up N’zoth quite a bit before losing.
She is essentially the max level player fully geared out that lets the lower level players win because she thinks it’s funny.
Legion was a good expansion overall with me just being turned off by the storyline being locked behind the mythic dungeons which were a throwback to vanilla dungeon runs in that you had to put quite a bit of effort into getting into one. They were prone to elitist groups when I left, and I just was not interested. Especially Karazhan which at the time was notorious for taking just about as long to complete as a raid run.
I think it was the expansion pack that would have been a good stopping point for World of Warcraft as a whole: The major bad group that had plagued the entire franchise is no more, both factions more or less won, there is new blood in the leadership roles, and there was potential for future games afterwards.