Easiest: Hooktail from Paper Mario The Thousand Year Door. I mean, it's impossible to NOT get the cricket sound badge. As soon as you use your hammer on his foot, you've basically won. And who falls for the ol' 100,000 coins trick? Or the super badge trick?* And who wants to smell his feet? >.< Even if he eats your audience (SICK SICK SICK!) to gain health after a while, he's still amazingly easy!
*Actually, if you accept the trouble at the trouble center from ???, it's actually Ms. Mowz and she sends you back to Hooktail's lair to get the super badge.
Coolest: Hmm..... I have multiple desicions. Cortez from PM:TTYD, since those floating swords were really cool, Armohgoma and Argorok from Loz:TP because eye lasers and smashing (literally) statues rule, and during the fight with Argorok my brother and I were singing "Spiderlink, Spiderlink, does whatever a Spiderlink does....." If you've fought you'd know what I was talking about.
Hardest: The Ice King guy from Paper Mario. For a long time, I could NEVER beat him! It was soooooo annoying.