Definitely the propeller suit or the fire-flower. Me and my two good friends have spent hours playing New Super Mario Bros. Wii. In fact, that's what we did instead of the Super Bowl this year.

I like the Fire Flower and the CAnnonballer cuz it shoots them out. Hard.

Tanooki Leaf most definitely. A very close second is the Cape Feather. I love both of them because they enable you to fly but Tanooki Leaf pretty much turns you into a very dope raccoon.

My all time favorite is giant mushroom(you know., like the giant orange mushroom from DS); however, I hardly ever PLAY the ds, so I don't really count it. I would have to say... Fire Flower(or Power Flower, whatever).

Personally I like the propeller cap, I've been saved by that thing many times. Lol. After that I'm going with the Fire mushroom, those things are fun! XD

The feather from Super Mario World because i like to glide all throughout the fun, easy looking levels. I also like the Ice Flower because I like freezing my enemies!

Either invicibility star, or fire flower. Maybe the wing cap.

I am going to have to say the Star Power in Mario Kart. So comes in handy at times.

In terms of hurting enemies? Penguin Suit. I liked Ice better than Fire. :p

In terms of getting around quicker? Feather. Was pretty easy to master and you pretty much have unlimited flight time until you hit something and you don't lose it right away if you're hit.

Mine would have to be the feather as it let me noob levels and just fly over them without getting hit and sometimes, only sometimes I would come new glitches such as warp whistles.

My favourite power up in Mario is the star power. Its my favourite power up because I am flipping invincible. The only thing bad about is that there is a time limit.

I liked the frog suit from Super Mario Bros. 3.  My favorite is the penguin suit from New Super Mario Bros Wii, though.  It was a blast gliding through the levels as a penguin.

My favorite Mario power up is the invincibility star. It's one of the first Mario power-ups and I've always enjoyed using it.

When I can't have the feather (Super Mario Bros 1 to 3, the one in 3 is very shabby) I adore the fire flower for its ranged killing power it has.

I have to say the Fire Flower, it's old, classic and I remember being a little kid and playing Mario, spitting out fire balls. 

my favroite mario power up is fire flower in super mario bros it fire at enemies like goombas koopa troopa para troopas  spinys bloopers cheep cheep hammer bros plants lakitus  fake koopas and real koopa  in super mario bros 3 one of koopa troopas  it won't work dry bone koopa troopa bullet bills thromp little thromp dry fish buy only stomp on bullet bills  boos  buzzy moles wigglers bomb bobs makikoopa   koopaling and shooting or wait only koopa stomps on mario in super mario world it's won't work  on but blocks work on   king boo charge chucks  little dino megamoles fuzzy lakitu  stomp on big dino rex  bowser in final battle but throws at him three times super mario rpg legend of the seven stars fire flowers only collect them all treasures racing 16 hidden hard to find mini games called flower tabs and flower jars collect them on boss fights sidequest in super smash bros 64 play mario or luigi you have fire at your rival thrown melee brawl special move inferno blast

For mine, I got to go with the cape feather from Super Mario World because I can pretty much fly through the level without dying.

I would say my favorite power up is the Invincibility star.

IgorBird122 : Yeah, but there is a flaw with that, you need speed for it and for vanilla dome 1 you have to go a long distance in a short amount of time, in which the spin jump is the best for this and after a while the levels have hidden bats so if you fly in that level you get hit.