Design-wise I'm absolutely in
love with Wilhelmina. Her Jekyll&Hyde-esque oscillations between
jolly church lady and
homicidal seahag are a thing of beauty.
For playing purposes, Scotland seems stronger than I thought. They simply
bleed Great Scientists and Great Engineers, the only reason why they didn't run away with the game in my first try is because they ended up on their own small tundra island. (in that game,
the Cree are running away, but hopefully I can beat them before they win science.)
But my vote went to
Mapuche. They are THE fresh breath of air in this expansion pack, have strong abilities in both Swift Hawk and Toqui, a reliable way of generating culture (Since theatre squares, as you all should know, have a pretty poor Culture output if you're not Teddy or either Greek leader), the gimmick of forcing enemies to lose loyalty as they lose troops is really fun and disruptive. Can't wait to try them in my next game.
My first insticts to dislike Korea proved correct because Seondeok is
terrible. Complaining about how you can't keep pace with her science while she has these
ridiculous advantages to science herself is one of my least favourite historical agenda flavours and her in-game personality is sooo effing snotty.

Definitely my second least favourite leader personality in Civ 6, only above that Brazillian blowhard.