Gengar, the final evolution of the original Ghost Pokémon, Ghastly. Gengar was already a threat and the mega evolution makes it unbelievably powerful. Before mega evolution, its Special Attack is 130 and Speed is 110, already pretty high. After mega evolution, its Special Attack goes up to 170 and Speed goes up to 130. This makes it a very good special sweeper.
Mega Gengar also gets the Shadow Tag ability. This traps the opponent so that they cannot switch to another Pokémon. The move, Baton Pass, will still allow the opponent to switch though.
It is a Ghost/Poison type, so it gets stab from some strong moves, such as Shadow Ball and Sludge Bomb. Mix that with immunities to Normal and Fighting type attacks, and he gets a well deserved spot in the top 5.
Another classic Pokémon with a much deserved mega evolution. Anyone who plays online has run into Mega Kangaskhan on more than one occasion. Kangaskhan's mega evolution is unique because it allows it to get an extra hit with every move, thanks to the Parental Bond ability. The child does the second hit at half damage. It also has a chance to cause any secondary effect the move has, as well as land a critical hit.
The real threat of this mega evolution comes from Kangaskhan's movepool. It can learn Fake Out and Sucker Punch, two priority moves. Power-up Punch is also amazing because you will get twice the Attack increase.
Choosing the right moves can make Mega Kangaskhan one of the best physical sweepers in the game.
With a high Attack and even higher Defense, Mega Mawile is already looking good. The mega evolution's ability really makes it shine. Huge Power doubles the damage of all attacks, so you can imagine how hard it can hit. It is also a Steel/Fairy type so it has a lot of resistances. Dragon and Poison type attacks can't touch Mawile. Considering Fairy type Pokémon are weak to Steel and Poison attacks, that takes away two weaknesses right there.
Mawile can learn the dreaded Sucker Punch as well, but it's power is stronger than almost every other Pokémon. It is a Physical attacker and gets STAB from Iron Head. It also gets that boost from the only physical Fairy attack in the game, Play Rough.
2. Charizard (X)
Normally, Charizard is my favorite Pokémon and the mega evolution for Pokémon X is one of the coolest. What makes Mega Charizard X so unique is the change from Special Attack to Attack. After mega evolving, Charizard's attack goes from 84 to 130. It also gains the ability, Tough Claws, which increases the power of all moves that make physical contact by 30%.
Mega Charizard X also gets the secondary type of Dragon. This allows it to hit very hard with not only Flare Blitz, but Dragon Claw as well. Give this Pokémon Dragon Dance and after a couple turns it will be nearly unstoppable.
The Terror of the Skies. The thing nightmares are made of. All of these accurately describe Mega Pinsir. Its Attack is a whopping 155 after mega evolving. It also has a high Defense and Speed. The ability though, this is what makes him King Pinsir. Aerilate increases the power of Normal type moves by 30% and turns them into the Flying type.
Since Pinsir changes to Bug/Flying when it mega evolves, it gets STAB from Normal attacks. Why is this good? Two reasons: Return and Quick Attack. Return has a base power of 102 and after the boosts it is 198.9. This move will one-hit KO pretty much any Pokémon that doesn't have high defense or resistance to Flying type attacks.
Quick Attack is a priority attack and gets a nice boost due to its ability. This is very useful in situations that Pinsir is not faster than the opponent.
The pure power of Mega Pinsir, when using the right moves, makes it the number one mega evolution.