I think he asks which Pokemon has the best Attack stat...
I think it's Medicham+Pure Power ability...Slaking has also pretty high attack.
Dont know about the legendaries...
Seriously though, a lot of people say it's Deoxys-F. It may be beaten by a Pokemon with Quick Attack, but if it stays in the battle it will sweep its way to victory and laugh at you.
Like I care though, cause i'll never get a Deoxys of my own.
If you base best by how much it's used, then that would be Earthquake. Everyone has at least one EQer on their team. If you mean Atk strength, that would be Deoxys (Atk Form). I don't know why someone was talking about Medicham (120).