Chirpster
Wobblewok
Gutsy Bones
Meganyan
Cap'n Crash
Eyeclone
Kin, Gin, Bronzlow
Dame Dedtime
Hans Full
Mallice
Kat Kraydel
Rebeus J
Hardy Hound
Hinozall
Teastroyer
Infinipea
Headasteam
Kabuking
Tattleterror
Mass Mutterer
Whismellowman
Robonyan 28
Akamaneki
Shirokoma
Inumaro
Nekokiyo
Nurarihyon
Shogun King
Norukasoruka
Mao Koi
Mao Shibu
Galactarian
Super Manager
Moso Monmon
Injaneno
Mr. Yamerunja
Gamegosu
Yoppu-kun
Betty-san
Kaminarasu Goro
Gogogo Godfather
Geborufushicho
Bishagatsuku
Suwa Daruma Shisho
Yatsuzaki
Dogushakkou
Chronomen
Gotekki
Daiyoki Hogewhale
Anahorimoguzo

S Rank:
Chairman McKraken (Two forms, each with a wide variety of different attacks, including the ability to use every elemental attack in the game. His main gimmick, in the form that's actually mostly usable, is based entirely around his hands, which gives him potential for a great Grab Game if you throw in something involving his tentacles.)

A Rank:
Dr. Maddiman (The second most recurring boss outside of Sproink. Dr. Maddiman's fight has you having to tap at the hearts he summons around the stage, which are spawned whenever he attacks someone. He can use the hearts to refill his health, which could easily be implimented as a sort of stacking mechanic in Smash. He also has a poison based attack, and some nasty looking scalpel claws)
Sproink (Just because the set I made for him was mediocre doesn't mean he doesn't have potential. He's one of the most recurring bosses in the entire series, and is pretty much the face of the boss Yokai. I feel like something good can come from him)

B Rank:
SV Snaggerjag (Weird body type, but excusable because he's a ghost and can float. His battle's gimmick has him fishing up different fish minions who have different powers, which could be interesting, and his fishing rod antenna could make for some interesting move applications, mostly in the Grab Game. He's also one of the few bosses you can befriend, which adds a bit more maybe)
Phantasmurai (Kind of seems like a generic swords character, and he mostly would be, but he has an interesting armor gimmick. Phantasmurai is actually a ghostly mech controlled by a mouse yokai, who fixes up the armor when its damaged. This could probably be used in a fun way, but msot of his set would be based around sword attacks either way.)

C Rank:

D Rank:
Slimamander (Super weird body type that doesn't work even though he can actually move around. It's weird. Only real powers are fire breath and hittings tuff with his head, doesn't even really have a unique inspirit power. He could have a gimmick based around moving where his eye is on his model, but that's not enough, I feel. Shame because he's my personal favorite and mechanically one of the best Yokai period.)

E Rank:
Tarantutor (I feel really bad for Taratutor, he's the only boss in the first game that you can't re-fight (besides McKraken's first form) so he's ultimately the most forgettable one, potentially in the entire series. He's a big spider with thunder powers for some reason, and his fight consists entirely of knocking him off of the ceiling to attack his fleshy underside. Its not exactly that interesting)
Massiface (Him Too Big. All his attacks are based around being really big and scary, which doesn't translate well into Smash outside of a boss. He would make a good boss, though)
Duwheel (Literally does not work, his model is that weird. The Smash side view makes it practically impossible, as Duwheel is a character who requires facing the screen in order for his gimmick to really work. His gimmick involves his attack pattern changing depending on which face he's rotated to, one of them is able to unleash devastating counter attacks)