My favorite power up is the white tannoki tail and my favorite flower design is the boomerang one --- Knowledge is knowing that a tomato is a fruit, but Wisdom is knowing not to put it in a fruit salad.
Flower design? ...uhh.. Fire Flower I guess. I liked it best in Super Mario World though, when it looked kind of like a tulip.
Favorite power? Fire Flower. It's powerful, but it's not over powered ( except against bosses in NSMB DS ) --- Complaints have a purpose. One voice becomes one hundred becomes one thousand. Enough common complaints among a crowd is sure to reach it's intended target.