I just finished unlocking every character and so far my favorites are Ness, Luigi, Toon Link, and Diddy.So far I've really enjoyed some of the others like Dedede and Snake, I just haven't put in the time with them yet to know for sure. Sonic really bummed me out though. I was looking forward to playing as him more then anyone, but I just couldn't get into it.

mewtwo.
mewtwo..
mewtwo...

This happened to me too. I unlocked Sonic sometime between Lucario and Snake. We were so suprised when his "challenger approaching" appeared. I've heard some people say it's related to how much you "run" during multi-player. This is an interesting theory, and IIRC, there was stat tracking for how much running you did, so it might be possible.  

MARTH! I loved Marth back in Melee too. He was and possibly still is my favorite. I really like Pit too. My preference leans toward any character with a sword.

Haven't unlocked everyone, but I'm still plowing people into the ground with Marth (thanks KIRK), including random people online.

I like Pit though, I might have to give him more serious playtime.

I didn't except to like playing Diddy Kong but he's awesome! I also liked Pit but I'm not as good with him.

Zero.

^That's who I wish was my favorite character.

Pit. He can fly, so you can gamble with him.

Olimar was the worst one in my opinion. Had a blast play snake. the crawl attack is awesome.(leopard print snake LOL!) The other new characters are aight.

I haven't unlocked everyone yet (only a few to be honest). ... but I do know I have to be changing from Mario. Having the FLUUD on his back (instead of the spin move) makes him less fun/less effective. He also seems to respond slowly. I had more success with Pickachu.

Mike from Morgantown