Video for every characters best stage https://www.youtube.com/watch?v=29c6-AJr8LY

Legal Stages

Battlefield
Final Destination
Small Battlefield
Dream Land
Smashville
Pokemon Stadium 2
Town And City
Hollow Bastion
Kalos

Slants? (sometimes legal, but people hate them)

Yoshi's Story
Lylat
Yoshi's Island

Very similar to other stages (often not legal, but are playable)

PS1
Unova Pokemon League
Wily Castle