Mega Salamence. A lot of people mock it and I personally don't see it for that. To me, it has a cool, sleek design that is very aerodynamic and makes a lot of logical sense. Especially since my fiance explained to me how its crescent wings would be very viable for cutting wind resistance and allowing it to fly smoothly. It just looks like this badass flying red moon and a lot more original and interesting than the regular Salamence, which I thought was nice, but nothing to write home to.
Also, Mega Audino is absolutely beautiful. Its design has been improved a ton and it's a Fairy-type! It just looks like an angelic creature wearing an awesome cape.
Lastly, Mega Rayquaza due to the fact that it looks like a boss from Final Fantasy. I could just see it being from Final Fantasy VI or something. Very intimidating, and much farther away from the more cartoony, less serious designs of Pokemon. I like how it just pushes the boundaries and makes us take the series more seriously. The original design of Rayquaza always just looked weird and clunky to me, not like a graceful master of the skies. Especially that horrible mouth that shows all of these gums. But this makes it sleeker and much more beautiful, more believable as a Legendary.