The Mario series has offered several power-ups through its 2D and 3D incarnations, and even some interesting takes on power-ups in its spin-offs.
So, I'd like to hear some of your faves, and please please please don't just list power-ups list threads are boring and don't engender discussion and they suck. So, yeah. Let's hear about those power-ups and what fun you had when you discovered them, and all that jazz.
Here's a list of all the power-ups from the mainline games.
I really like Boo Mario from SMG/2. It has a great floaty feeling to it and being able to go through walls is a fun ability. I like the look of it quite a bit, too. Doesn't hurt that the levels it's featured in are very memorable.
I suppose in looking over the list I have a weird soft spot for Bunny Mario, obtained with the Carrot power-up. It feels kinda of janky to control, but intentionally so, and in a game filled with offbeat quirks, it fits right in.
The cat suit is just a genius power-up as it does for the 3D space what the Tanooki suit did for the 2D space - it allows interaction with the world in a way that accommodates mistakes while simultaneously also assisting in top-notch platforming. So it serves as a safety net for players and also as a true power-up that imbues some extra special abilities to pull off some neat moves. The Lucky Bell is even more special, as it offers invincibility.
The Penguin Suit is also a fave. Slip-sliding through levels is delightfully fun, especially on a time trial in NSMBU.
(Let's keep this thread clean of Odyssey details for any and all who are trying to go in as fresh as possible.)