Lucs has replaced Mewtwo as my main as well, but honestly I thought I would be maining Sonic. I absolutely fangirled it up when Sonic was announced, mostly because my friends kept telling me he would never be in a Smash. When Lucario was announced I barely blinked, but I did think "Oh, I guess Mewtwo's not coming back."
My mind wasn't really made up until this week, after I reserved the game and watched some good fights. Honestly, I'm maining Lucario because he is so mind-numbingly graceful. You can tell he's based off martial arts and he does it well. When I watched Lucario fight, and then watched Sonic, I was terribly disappointed with the blue blur. Spindash, and some kicks and a neat up-b . . . huh. No thanks, give me the sexy kung-fu jackal with dreds. Also AURA!