Ok, here's my top 5.
1. Booster (SMRPG). The greatest nutcase ever.
"What's this...? Water... coming from her eyes? Are you leaking, my dear? ... Tastes salty. This stuff must be corrosive! It tastes like the sea!"
Need I say more? Likewise, my favorite Roman leader was Caligula. (He appointed his horse as a consul. Enough said.)
2. Gargantua Blargg (Tetris Attack, YI) The BEST mascot in TA. Well, they're all the same, but... You GOTTA love the ARRRRRRRGH noise he makes. You just HAVE to. And I like the music for his background, too. Although his role in YI is rather minimal (I think he appears only once, in 1-4)... He's still cool.
3. Black, White, or Orange Yoshi (SMS, YS) All the neglected colors. iViva la revolution! Errrrr, never mind.
4. The Happy Mask Seller (Majora's Mask) The only Non-Mario character up there, this guy is hilarious. Just the way he's presented... One minute he's smiling, and IMMEDIATLY (In about the time it takes to blink) he's choking Link, and then after another similar transition he's begging for Link to help. It's funny. REALLY funny, if you ask me.
5. Luigi. The single most neglected Mario character ever. Yes, there was games like Luigi's Mansion... but there should be a series of Luigi games, you know, like the Yoshi series spun off? (kind of)
There you have it. Interesting top 5, huh?
Everyone has their own opinions, no matter how WRONG they are.