Well my favorite was lucario because he was my favorite in sinnoh and his mega is beast lol! But now altaria is my go to fluff ball i love it so much its got a great typing and it takes out the majority of my foes!
That being said its great because i need to level up lower level pokemon i send in altaria and the exp share probably does more work than altaria has to do its just 1Hit KO with her. I use it in pretty much every battle to show off i just imagine myself going oh u got a level 5 pokemon MEGA EVOLVE AND KILL IT lol
To comment on the design of Megas i think the largest thing to see is th change in appearance usually either in defense and attack where its generally got more muscle for ex. Swamperts Mega have u seen that thing? I mean its overly buff! Since you did post this dragon why not tell us your favorite mega and such XD
Definitely agree with you on Altaria. Its bulk, typing and recovery allow it to get in quite a few dragon dances before going all out offensively. Dragon/Fairy is an amazing type combo offensively as well as defensively, Pixilate-boosted Return sweeps teams! Mine OHKO'd Steven's Metagross with a resisted hit, which just goes to show how amazing Mega Altaria can be.
Another favourite of mine is Mega Absol. Absol has always been one of my favourites since I played RSE as a kid. Mega Evolution just gave me more reasons to love it, as I can now use it online. While it's as frail as they get, M-Absol is a pretty good mixed sweeper and wall breaker.