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Mega Mewtwo X, man. Lucario takes x2 damage from both psychic and fighting. The stats for mega evolutions are still pretty elusive but just compare their regular forms at max. It's in the numbers. Mewtwo is also known for one of the largest move pools. I don't think it's fair to compare the two, to be honest.. but you asked..
I assume your bias falls towards Lucario because of the picture, but it's just not even close...
Mega Lucario does have adaptability though. It's a great ability. Keep pushing that.