I like the combination of speed, power, and range that Mewtwo has, it feels like I'm actually playing as a legendary, with amazing power, they had to reflect that base 154 special attack stat somehow. Also, ever since I saw Mewtwo returns, I have liked the character, and he has been my favorite legendary aside from Marshadow, since I saw that.
When I was little, I really liked Monkeys, when I got brawl because blind parents don't notice army man with big guns. I saw Diddy kong, and I immediately knew I was gonna main him, unknowing of tier lists at the time, I used him all through the subspace, then at tabuu, when Sonic appeared, I was all "OH MY GOOAD SANIK UUGGH" and I added him to the 6 character roster to fight the stupid jank boss, my final roster was, oh lord, Diddy, Sonic, Wario, Ganondorf, MK, and Falco. In smash 4, I learned of the sacred death combo, hoo hah, I had my first big advancement as a player as diddy kong, the advancement that seperated the noobs from the advanced noobs that actually know combos, this repeatedly made me advance more and more as a player.
I started using Wario because when I was young I really liked the Wario land games, I liked them so much that I bought Wario land shake it, and immediately fell in love with the graphics and style of gameplay. I liked him so much I used him in brawl, and I also picked him up in this game back in april 2015, just to see what he was like. I liked the idea of a heavy that is small and hard to combo.
now for the crappy reasons.
I main cloud because as a child I loved the idea of a character wielding a weapon bigger than themself, so when I saw cloud I was all "UMG BIG SORD GUY" with the typo. When I saw big sord guy with the typo in smash, I picked him up, and I started playing FF7 in the process.
I play Sonic because, like every one else not named wrath, and fg sonics, I loved his games and fast. So I main him because fast.