As a kid who got Mario 3 on release day, and just for the rarity and awesomness of being able to kill just about all enemies including ghosts, thwomps and everything in between, I always had a love for the Hammer suit.
That is not to say it doesn't have it's drawbacks. I think the most attempts you can get at finding the little hammer brother doll is 3. Ive never found more than 3 in any run through. One of the major drawbacks with this item is you can't slide down a hill with it, and it seems like it is real easy to lose once you have it. Most of the time I would try to slide down a hill, go into beetle mode and get touchd by an enemy and lose the powerup. Takes some skill to aim the hammers cause it functions like a mortar launcher with such a high angle trajectory. Killing Bowser takes something like 5 or 6 shots, but getting there intact is another story.
fanboyism aside. I think the Racoon Leaf was the overall best powerup from Mario 1-Mario 64 as there was such a wide range of stuff you could do with it. Tanooki was basically the same thing but you could use the statue feature to let enemies walk past you or crush the harder guys by jumping above them and turning into stone to squash em.
Least favorite. That fucking winged cap from Mario 64 was rediculously hard to maneuver and gave me bitch fits while trying to be a completionist and collect all the stars in the game.
Kuribos Shoe should of made more than 1 level appearance in Mario 3. It was fucking awesome being in fire mode and having the use of the giant boot of death while shooting fireballs of damnation at every enemy.