I can't decide on a Physical or Special Mega Altaria since it has equal stats in both Special Attack and Attack and also a pretty good movepool for both. If you've had better experience with one more than the other competitively please let me know. Thanks!
Ive personally have had better luck with physical MAltaria.
252 atk 252 spe 4hp
Set up a few Ddances and it sweeps. Return with Pixilate gives it 132 base power and gives it STAB fairy type. Earthquake counters Poison and steel types that would resist Return and Roost heals it up from hits taken as it has great natural bulk to take them.