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Based purely on design.
M-Slowbro is the best example of what a Mega Evolution can be - it takes an element of the original design to its logical conclusion. The Shellder which originally bit Slowpoke's tail now protects the body from harm.
M-Sableye isn't much of a visual jump as many other Mega's, but I think it works really well here. Sableye rips the gem from its chest to act as a shield. That's pretty badass. And then it eats it before reverting back to normal! Amazing.
Mega Sceptile is just what Sceptile should have always been desu. An improvement on the original design in every way.