I would have to say the Gold Flower, it's provided so much fun in going for high scores in Coin Rush Mode, as well as sheer destructive power. It has utility too switching brick blocks to coins, which has interesting effects when a P-switch is in effect.
Otherwise I would've said the Blue Koopa Shell is my favourite. There's something I like a lot about becoming the bouncing Koopa Shell. Fun fact: Spinning in a Blue Koopa Shell has a lower top-speed than just running normally. --- Clean Cut high score: 65 cuts? http://www.youtube.com/watch?v=T6_Ty50Kais