Ultra Balls, Dusk Balls, Quick Balls, and Timer Balls because they're all extremely useful! Ultra Balls have a better catch rate for any Pokémon. Dusk Balls have helped me catch a bunch of legendaries in caves, along with any Pokémon I encounter at night. Quick Balls make for good use when I run into a Pokémon I really want but am too high leveled to weaken it. Timer Balls have helped me for when a battle has gone on for too long and don't want to waste the rest of my Ultra/Dusk Balls. (:
For design, I like all of them and Luxury Balls, as well; Timer Balls might have to be my favorite, because when you send out a Pokémon or they break free, a little timer appears for a second and it looks really cool! :D