Honestly mega Audino wasn’t the best, but it was ok, hear me out.
I do agree that soooo many other Pokemon deserved a mega evolution over Audino, flygon included, I honestly have no idea why Audino was chosen. However, in competitive you’ll find that if used effectively (especially in double battles) Audino can be a decent support Pokemon. Audino really isn’t the best attacker but in double battles it is a good support, although i’ll Admit that Pokemon like blissey and even gardevoir a way better, but it’s always a nice surprise to have a competitive audino/mega audino come and confuse your whole team with sweetkiss, heal itself with draining kiss, and use substitute instead of just fainting! (Sorry I got really beaten down by one recently), and it has a decent hp.
Really though this is just a matter of opinion, and as much as I might want to sometimes, I can’t undo the fact that audino has a mega evolution, so we can just hope that other Pokemon have mega evolutions in the next gen.