Personally, I main Greninja because he just feels the most rounded to me. He's got several amazing disjoints, incredible speed in both the air and on the ground, sexy AF combo game, and a surprising amount of kill power. He may not be quite as fast as sheik in terms of attack speed, but his kill power is leaps and bounds over hers. His projectile is super versatile, ranging between a fast stun and a slow death ball. Then there's his recovery. As others have mentioned, he can reverse-edgeguard all day with SS, and coupled with Hydro pump and Substitute, he can come back from almost anything. I'd honestly argue he has one of, if not THE, best recovery kits in the game. Then, to top it all off, he just looks soooo fricking cool. His design is sharp and sleek while his moves are flashy and satisfying to land. It's like driving a Ferrari with machine guns that pop out of the head-lights. And finally, to put a topper on the tippy top of it all, he was one of my favorite ass-kickers from Pokemon X and Y. THAT is why I main the ninja frog.