Mega Aggron and Houndoom.
Of the two, Aggron is the most consistently useful. I use the same set as
>>16413279 except I go with Autotomize and Iron Head; flinches have won me the game.
I like Houndoom's design the best, and it's fucking hilarious when everything goes just right and I decimate 4 of their 6 Pokemon with a Flash Fired Nasty Plot in the Sun. That just doesn't happen often if I'm playing against somebody good.
But man, when my Forretress lead uses Sunny Day, those bad players on Battle Spot rush to use fire attacks, despite team preview.
I'm really happy that my Gen 2 favorite got a Mega that I can make work.