Superyoshiom
Smash Master
I know what you're thinkin: Why have Mew when you could have his clone, Mewtwo? Well, you're probably right, but you never know...
Mew is one of the most famous legendaries in the Pokemon franchise and it's DNA was used to create Mewtwo. In the games, it is very powerful and can learn every TM in the game.
Moveset:
Neutral B: Psyshock
I imagine this move being similar to Lucario's aura sphere or Samus's charge shot. You can charge up an energy ball for more damage and knockback.
Side B: Mega Punch
Mew rams into its opponent for some decent knockback.
Up B: Teleport
Pretty similar to Mewtwo's teleport from Melee.
Down B: Metronome
Like in the games, this can either be extremely powerul, or a dud. The most this attack would do is around 50%.
Final Smash: Psychic
Mew controls all characters at once with telekinesis and can move them anywhere in the stage they want. A bit overpowered, so I'd imagine some minimal range to grab characters at first.