I think the best way for me to answer is by generation:
Gen 4 part 2: Arceus, curious to see how that goes especially since the plan is to make her more complicated to capture.
Gen 5: Meloetta, I'm curious to see how the ai generation would work on her since she already has a feminine design. Also being a mythical usually means Chainsaw gonna get wacky with exploration or battle/quiz.
Gen 6: This is hard because there is so few choices but Hoopa or Xerneas because Hoopa got the hoop gimmick which seems fun to play with while for Xerneas in the game about breeding, the Life Pokemon has got to have some... interesting perspectives.
Gen 7(non ultra beasts): Necrozma, primarily because of the challenge it presents. That's been fun seeing with this game having to wrangle the story around the ai being finnicky.
UB's: I wanna be crushed by big bug lady legs. (Pheromosa)
Gen 8: Eternatus, not trying to be a trend chaser but I do wonder how we will handle da BIG.
Gen 9(non paradox): Ogerpon because it genuinely seems like the most interesting legend with its own legend in that poor poor game.
Paradox: The paradox version of The Swords of Justice because the other options kinda silly looking like ancient Raiku a giraffe
