Mine are the Hisuian balls, specifically the Hisuian ultra ball. I just love the design of the latch, and the color palette used for all the balls. I especially enjoy the little flourish on the yellow stripes of the ultra ball!
What's your favorite? Any particular reasons why?
The level ball is particularly nice.
I am also a sucker for the basic pokéball. I think it's the ball I use the second most, right after ultra balls. And there's always something so satisfying about catching a late game 'mon in a basic pokéball. No frills, no special bonuses, just me, the 'mon, and a good ole pokéball as Arceus intended.