Kirby is my favorite.  Not sure which of the 1000s of characters is my least favorite.

Favourite:

Link and Kirby

Most Hated:

Mario

Samus is the best playable character, but...

As for the worst, Tingle is the Jar Jar Binks of Nintendo.  Or maybe the lesser members of the Donkey Kong family.

If we are just talking about characters and not neccesarily the games I think my favorites are Yoshi (playing SMG2 again, and every time he says Yoshi! it is adorable). I also think Captain Olimar from Pikmin is awesome, he is very funny looking and I like his weird journal entries.

I don't really have a character I dslike, but I think Princess Peach lacks personality considering how many games she is in/ how important to the Mario universe she is.

From a design perspective I really liked Light Suit Samus..

and really hated this Pianta..

Favorite: Link :D

Least: Wario :/

Wow that looks beautiful

It's one of my favorite Samus fan-pics as well, here's the artist's page - http://raydiant.cghub.com/

and this is what the suit looked like in official art.

favourite:

Least favourite: