Most effective mega evolution

I feel like Blaziken is being able to Baton Pass boosts while also being a great sweeper by himself.

I really like Mega Gyarados too. Intimidate with regular Gyarados and then Mega Evolve + Dragon Dance is great. Mega Gyarados is pretty bulky too which helps him even more.

 

Mega Mawile gets Huge Power. Combine that with the right IV's, EV's and Nature, and it can reach an Attack stat that would equal 259 base power, 255 being the maximum. It litterally surpasses the limit. Not even to mention that it learns Swords Dance! And despite all this, it still keeps really decent Defense and Special Defense. This thing can sweep. Real hard.

 

Manectric, when used somewhat as support, can be really great. He is good on switch-ins with lightningrod, mega-volving with intimidate possibly volt switching and intimidating back in, overheat then volt switch, and also setting a light screen up as well.

 

I'd say Scizor, at least out of the ones which haven't been picked yet. It's really good, even though I haven't gotten a chance at using it, it's a huge threat to most teams, and a good/perfect counter to Mega Gardevoir.