At a casual level he's still pretty good due to the fact that he has a strong KO move with no startup frames (down smash), four recovery moves (as well as five jumps and a glide), and his neutral B (mach tornado) has priority over most moves and he can't be knocked out of it.

At the highest level, he's the best character in the game because of the above reasons and the fact that most of his moves are near-instant and with little cooldown so there is little risk if you miss. He has the most options in any given situation, up until recently he has been the most used and most successful character in tournaments. He has the best matchups in the game as well (obviously) His up B (the one where he spins in a circle then glides) is both lighting fast, has super armour for a few frames (means he takes no knockback) and has such extreme knockback you can KO most characters at around 40% provided you're both airborne.