Raccoon, Hammer, Tanooki.
Seriously. First time I played SMB3 and saw Mario grow a tail, I was thinking "Whoa. Cool." That image still exists in my mind today.
The Tanooki suit is self-explanitory. "Whoa, man! He's a raccoon! That turns into a statue!"
And the Hammer suit is just fun to mess with. And Mario throws hammers. Which makes for numerous (well, it's usually the same one over and over again) MC Hammer jokes.