I really like the concept behind both Mega Mewtwos, both cool takes on Mega Evolution letting Pokemon go beyond their limits
Mega Mewtwo Y puts emphasis on the fact that Mewtwo is a clone of Mew, and going beyond would just mean resembling the original more (but still being an enhanced, perfected version of the original)
Mega Mewtwo X instead just turns the "man made fighting machine" aspect of Mewtwo up to 11, hence why it gains the Fighting type and it in general looks sturdier and more battle-ready

What are your favourite concepts/designs when it comes to Mega Evolutions?