Yeah I usually go for surprise Shadow Balls, fair, and Usmash.
This is off-topic but love the profile picture. Really looking forward to playing Splatoon. (Which by the way, the demo is gonna be available today at 8-9pm PT, incase some didn't know.)
The thing I appreciate about Mewtwo is that he teaches me a defensive game, which i have to learn to play when as Lucario in high percents. The shadowball animation is really quick and sometimes, I throw my enemy off-stage. Jump off stage to act like i'm going to do something but end up just jumping right back on it, which usually they follow me and I get a pointblank SB. And with such good KO power, easy...90%? On medium weight.
Fair is very quick so I do tend to use it a lot when challenging my opponent's aerials, because usually even if I get hit, they'll get hit too and even managed to be KO'd.
And Usmash is godly compared to his other two. Anyone around 100% is well within a kill-range. Which I did find out, if you get the front of the Utilt, it sets them perfectly above you to easily land Usmash.
Though i'm sure you probably knew that. XP
Still no where decent with him but best to know him than to not know anything at all.