For me, non other than Son Guko
Reason why:
He always beat most powerful villains in Universe.
He can multiply his strength 100x or more per level.
He can even destroy a planet(planet busting abilities if he has will to do so.
It was stated that it takes 200-300 kilis to destroy a planet. Since they never specified how large of a planet I'm going to assume (since they were on earth when it was stated) that they meant an average size planet around Earths size.
Goku's Ss1 can destroy: 10-15 planets
Goku's Ss2 can destroy: 20-30 planets
Goku's Ss3 can destroy: 80-120 planets