I think they're all super cool.
When the game got it's first trailer, I was a little concerned about Mind Flayers being marketed as the big bad, because the first two games were carried by the personalities of their respective villains. So how could a monstrous nameless mind flayer even hope to compare
(obviously there was more to one in particular now that the game's out)
.
There was also the question of what makes a Baldur's Gate game if it doesn't involve the Bhaalspawn? I talked myself into being satisfied with it taking place in the Sword Coast again with the Bhaalspawn Crisis being part of its history.
But now we know this is essentially an evolution of the Bhaalspawn Crisis on a much bigger scale and I think that's awesome.
I'm not going to pretend to be a narrative genius, so I don't have a lot of thoughts on the writing. But it feels like we got a theme-fitting legion of doom with great designs and great voices(largely what made Sarevok and Irenicus)
So yeah, I love it.
In order of preference, Gortash > Keth > Orin
But it's hard to beat that initial introduction scene with Ketheric. That was on the level of meeting Sarevok in the wild with Gorion in my eyes.