If you know me well enough, you know that I have a certain...distaste towards Mega Evolutions, but I'm not going to go into that here. My favourite Mega Evolution is definitely Mega Houndoom. One reason is because it's a very powerful Pokemon, but not ridiculously powerful like many other Megas. Also, unlike far too many of them, Houndoom actually deserved and need a Mega Evolution, since, before Gen 6, Houndoom was an incredibly lackluster Pokemon. Finally, let's talk about the design.
I love it! It looks completely cool and badass, but also doesn't change vanilla Houndoom's design too much. I think it makes just enough aesthetic changes to make look especially intimidating and cool to look at!