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?
In the attached pic? It's a Heavy Ball. Has a higher catch chance if used on Pokémon who haven't noticed the player.
EDIT: It's a Hisuian Heavy Ball, from Legends: Arceus. I have been informed there is also a heavy ball in the mainline games, and they are for catching heavy Pokémon.