It really just depends on the level as far as which booster I like best... they're all pretty level specific needs wise. I suppose if I had to choose one, it would be the candy ball thing with the sprinkles since they're good to have in any level I get stumped on.
I hate having all the boosters at the beginning of the game because they get used up right away, there should be a way to select them when you want to use them during the game.