Mewtwo. Nuff said
I'll say Kyogre. 35.9% usage. Beats out Mewtwo by about 3.7%.
I have to say Dialga. Dragon/Steel combo leaves only 5 attack that aren't not very effective. Fire, Ice, and Dragon moves are just normal damage while only Fighting and Ground give super effective hits and since Dialga can be taught magnet rise that eliminates the Ground weakness leaving only 1 super effective and 3 regular moves that will actually bother it. It also has good base def and s.def so unless you can hit it really hard with a move that it doesn't resist you won't KO it while with it's base s.att being high, having a 150 base power dragon type move with stab bonus means that unless you are part steel type you get KOed. Also Dialga can learn Thunder so Kyogre's drizzle get's turned against it. And with Earth Power it gets ground coverage on the steel type legendaries it can't hit hard with Roar of Time. So Dialga wins for most powerful.
I have to say Dialga. Dragon/Steel combo leaves only 5 attack that aren't not very effective. Fire, Ice, and Dragon moves are just normal damage while only Fighting and Ground give super effective hits and since Dialga can be taught magnet rise that eliminates the Ground weakness leaving only 1 super effective and 3 regular moves that will actually bother it. It also has good base def and s.def so unless you can hit it really hard with a move that it doesn't resist you won't KO it while with it's base s.att being high, having a 150 base power dragon type move with stab bonus means that unless you are part steel type you get KOed. Also Dialga can learn Thunder so Kyogre's drizzle get's turned against it. And with Earth Power it gets ground coverage on the steel type legendaries it can't hit hard with Roar of Time. So Dialga wins for most powerful.