I'd take Shovel Knight. An old-school Pac-Man wouldn't be shabby, I suppose. Third party developed/first party published characters from Cing would probably make me cry tears of joy. Ashley Mizuki Robbins or Kyle Hyde would be heavenly, even though they're so niched they'd have no purpose as Amiibos.
Why wouldn't we get Sonic? I assumed they were going to do all of the Smash characters right? Unless some aren't going to have their own CPU fighter counterparts?
I don't know that I'd be interested in many third party amiibos. The thing that works with a Link amiibo is that there could be bonus Zelda content in a ton of games. I don't really expect the same out of third parties.
From just a looks standpoint? Mega Man X and Zero would be awesome. Proto Man as well.