Easiest: Bowser, ML:SS... the very first battle. You can't even lose that fight... does it really count?
Coolest: By all means... MACHOOOOOO GRUBBAAAAAAAAA! And the Titan Dweevil, except when it kills my Pikmin T_T EDIT: Oh yes, Popple is cool too, SEE?! And Fawful. By all means, Fawful.
Hardest: I can never seem to kill Phantom Hourglass's final boss. Maybe it's just my crappy DS. Also, Bowser from SM64 was pretty tough, and that Empress Bulblax in the Hole of Heroes... well, you start out at the end where the Bulborb Larvae come out. Which can digest Pikmin in about two nanoseconds. Also, the Rockfalls! When the Empress hits the wall, rocks fall down and make flowery pancakes out of your horde. (Don't worry... Hole of Heroes is the only place this happens.) And let's not forget... you may forget to whistle off your pikmin while they're attacking... which means... BAI BAI PIKMANS. So don't even call it easy.