Well, as I said, they all have awesome design, but if I had to pick top five, it would be something like

Mega Gengar
Mega Charizard X
Mega Mawile
Mega Rayquaza
Mega Aerodactyl

In no particular order, because it might change any day. At first, I was a bit sceptic when they came around, since I thought about the concept being like in Digimon where the monsters only evolve to win a certain battle, but, eventually, I accepted the idea.