This is actually a fascinating question to think about and pick apart.

I think if I were to say non-OC wise, it'd probably be the Sinners (Particularly Heathcliff and Don Qui) from Limbus Company; they've got such interesting and fun mannerisms to pick apart, as well as they have fascinating speech patterns as well. That being said, I also think Faust is pretty fun to write! Partially because there's a lot more nuance to her actions; but the impulsiveness of Don Qui, and the underhanded scheming when he's got an idea in hid head (and general mannerisms) of Heathcliff can be extremely fun. I think another thing that lends to it ihave. Established rapport and dynamic they have.

As for OC content, it's hard to toss up between Bianca and Heather; but I think I'd fall back on Bianca. They've both got a nice smug quality about them that makes them a blast to write, but Bianca has a quality of hubris and overconfidence, which is extremely fun to write, and that I want to explore soon, whereas Heather is more the scheming schemer. I think they both have their merits, but a scientist so lost in the sauce that her hubris gets the better of her is extremely fun. She's also got a fun dynamic with Lychee, which mirrors Heather's dynamic with Ace, but as I have more control over Lychee (as Ace is the PoV character, and You), things can get a little spicier... But I haven't shown any of that off yet.

Fascinating question though, and I am going to think about it for hours.