If you got any Mario power up what would you want and why

i want to turn into a car

Hmm....Fire Flower has always been a mainstay favorite, but I always loved the Racoon leaf or Tanooki suit. The White Tanooki suit from 3D World is definitely awesome and became a new favorite of mine considering you're invulnerable for the most part.
I'd choose the super crown

Just kidding, If I couldn't choose a permanent invincibility item, I'd probably choose the cape feather, It has a variety of flight and defense options, and getting a cape is a neat bonus.