While having VERY bad bulk, it makes up for in speed. 125 is highly respectable and outspeeds pretty much everything ingame except Deoxys. Attack isn't great, but still usable to a certain extent.
High defense and OK special defense. 80 attack is usable and sturdy is a great ability to have. Available just before Falarbor town.
Mega Evolution is crazy powerful with Dragon Dance. Just really good. Nothing more to say here.
Final choice is up to you although I would go with Altaria.
Hope this helps!