Currently, I would think Mewtwo would be the stongest Pok?mon of all. Comon, it (probably a he) has the power to completely negate any attack and also is capable of wiping out life on Earth. Even Mew didn't hurt Mewtwo much. But then again, maybe Mew didn't use it's full power...
In the future however, I'm very sure that Pikachu would become much, MUCH stronger, maybe to the point where its power is equal to that of Mewtwo. Ash's Pikachu is no ordinary Pok?mon. Right now, Pikachu can demolish small mountains with it's Thunder attack. At the rate that Pikachu's strength is growing, it can take on the other legendaries. If you guys haven't noticed, all of the legendaries that Ash has encountered in the movies that he order his Pok?mon to attack, only Pikachu was able to phase them: able to counter Articuno's, Zapdos, and Moltres's attacks with the help of Charizard and Squirtle; actually phased Unown's Entei. Cyndaquil, Totadile, and Charizard fought valiently, but didn't actually damage Entei in any way.
