It's a toss up between Mega Mawile and Mega Lopunny. They look amazing and their designs are stellar. Also, I love Mawile and Lopunny so much ahh!!
I was never fond of their designs since both Megas look the same minus their eyes. What I've always wondered is why Mega Latios and Mega Latias had their data in X/Y even though they didn't get a proper debut until OR/AS.Aside from them, probably Mega Latios/Latias for the soaring feature.