In melee, there's a reason why I play- no, main Mewtwo.
He's unpredictable. Other than unknown match-ups, Mewtwo can teleport (completely lagless when in the air) and has moves like confusion and disable others don't see coming. Sure, not the most useful moves, but they can be handy at the right situation.
You can't disregard his positives. Mewtwo has an amazing tilt game (up-tilt is very nice for opponents appearing from below, including up-air), has absolutely glorious recovery unlike the majority of the viable cast, (which yes, is counterbalanced by being light and big, but can make for great edgeguarding ability), has a double-jump cancel, which is useful for landing aerial moves, and has a really good wavedash.
Also, side-b on battlefield? Gonna drag them into the abyss!
Finally, He's fun. You know, there's a certain joy when you get tired of seeing the same 8-4 characters on the screen battle it out for more than a decade straight and then you actually see a low-tier on the screen. I also have a Falco, but things get more serious when my Falco goes against another high-tier character. Most likely, they know the match-up. When I play Mewtwo, I can't help but smile when I see what is considered one of the worst characters in the game playing against someone serious with Falcon or Fox. And my opponent hardly knows anything about Mewtwo besides whatever's infamous. It makes for a mindset that is unpredictable and happy! You get what I'm saying?
I play Mewtwo to find what I can do with him. I tend to not do what everyone else does when it comes to smash. Mewtwo has great things, you can't just focus on the bad stuff the moment the character is on topic.