My favorite Mega Evolution is probably Mega Blastoise. Blastoise is my favorite final-form starter evolution due to its design, so Mega Blastoise getting Mega Launcher as its Ability is really neat.
Most other Mega Evolutions are pretty good. Other favorites are Mega Venusaur, Mega Beedrill, Mega Pidgeot, Mega Alakazam, Mega Kangaskhan, Mega Pinsir, Mega Aerodactyl, both Mega Mewtwo forms, Mega Heracross, and Mega Sableye.
Both Mega Charizard forms are good in their own ways, but I prefer Mega Charizard Y both in utility and design. Mega Houndoom is absolutely terrible as far as I'm concerned, but I love the look of it. I like Mega Ampharos, Mega Audino, and Mega Lopunny for adding dual-typing to them and, in Audino's case, actually giving me an excuse to have used it on a team. Finally, I want to give a mention to Mega Altaria, Mega Gardevoir, Mega Glalie, and Mega Salamence for their Pixilate, Refrigerate, and Aerilate Abilities.
As far as what I consider the worst Mega Evolution, while I mentioned that I love its design, I have to say Mega Houndoom. I really do not like Solar Power as an Ability and without actually engaging the beneficial half of the Ability, it replaces Flash Fire which is far more useful. From a design standpoint though, Mega Latias and Mega Latios are pretty disappointing since they look nearly identical.
