I love a lot of Kirby's hats but some of them were just so fun and creative, so I'll mention those.
Fighter (various games) - With all the different ways to punch and kick with this ability, it's an obvious choice in practically any Kirby game you're playing. It was tons of fun in Super Star and future games expanded on its idea, especially Amazing Mirror and Squeak Squad.
Ninja (Sqeak Squad) - This hat was a whole lot of fu in Super Star witn its little suprises that made you realize that you only thought you knew what you could do with this hat. But later in Squeak Squad it made an appearance and somehow they managed to add a few more suprises to this.
Crash (Super Star) - When this first made an appearance, it was a one-time mega explosion that did massive damage in a single explosion. When it made its reappearance in Super Star, Kirby mind games'd everyone by getting ready to explode only to fall asleep. Just when you were wondering if anything was going to happen or not, Kirby finishes the explosion and blasts everyone to pieces.
Plasma (Super Star) - This Hat is just hilarious. It is really weak unless you charge it up by running around, at which point its capable of shooting out ginormous blasts of electricity.
Spark + Cat (Kirby's Adventure 3) - The animation for this is so funny, and was a precursor to the Plasma hat I believe. It involves Nago the Cat rolling Kirby around. This doesn't seem to do much until you realize that if you charge up Kirby with static electricity, he'll turn into a rolling ball of electric destruction!
Broom + Cat (Kirby's Adventure 3) - I also have to mention how silly it is to see Kirby turn into some sort of towel and watch that fat cat, Nago, ride around on top of him. XD
Smash Kirby (Amazing Mirror) - What a suprise to see this in a Kirby game. It was funny, and being able to use multiple abilities with a single hat vwas pretty useful.
Freeze (Nightmare in Dreamland) - an homage to Ice Climbers, no doubt. Funny.