While Black and White tried to recreate that feeling of playing Red and Blue by limiting you to only Unova dex Pokemon up until you become the champion, X and Y has older Pokemon pop up alongside new Kalos dex Pokemon, and for good reason. The new mega evolutions aren't new National Dex entries but purely apart of the new battle mechanics. That's why older Pokemon will be included in this list for their mega evolutions and the newly retconned fairy Pokemon.
Aggron has always been my favorite Pokemon since it appeared in gen III in Ruby and Sapphire. While it has a cool design and ridiculous defense, it's general outline is ultimately too niche and extreme; dual-typed as steel/rock gives it two maddening 4x weaknesses to easily accessible fighting and ground type attacks plus something as simple as Thunderbolt would probably do more damage against Aggron's low end special defense.
With the advent of mega evolutions though, Mega Aggron looks to rectify its past mistakes and stand tall as a brand new steel beast. It loses its 4x weaknesses by becoming pure steel, its attack, defense, and special defense go up, and as if it wasn't bulky enough its ability changes to Filter which powers down super effective attacks by 25%.
All this adds up to be the true form of what Aggron should be: a monstrous wall that will tear you down before you can. All hail the steel beast!
When Honedge was first revealed, it quickly became a novel idea. A ghost that's also a sword. A steel Pokemon that's also a ghost. It's predictably very strong on the physical front while its typing and defense offers easy switch-ins.
After becoming Doublade though, it can evolve again through the Dusk Stone and it becomes Aegislash and that's where things get interesting.
Aegislash can shift between a defense form and an attack form by using its signature move, King's Shield. Not only does it act like Protect and reduce an attacker's offensive strength if they clash with it, it causes Aegislash to shift into defense form after attacking in attack form. Its stats shift accordingly, as it can turn from a sturdy wall to a terrifying attacker on its turn. Slowly but sure, people will learn to anticipate what most Aegislash players will do which is where the interesting mind games come in.
Aegislash will inevitably only have one or two move sets. It needs King's Shield for sure and its speed is horrible so most will probably use Shadow Sneak, a priority ghost attack that moves first. Playing Aegislash and fighting against one will become 100% prediction based. Will he use King's Shield now? Will he attack now? Will he use something different to move last? Even if you know what Aegislash can do, there's still a massive mind game to wrap your head around!
Hawlucha is a uniquely eagle-themed luchador. He's fairly straightforward sweeper with high attack and speed. His ability also begs to use Acrobatics, a flying attack that doubles in strength when not carrying an item. So rather then not carry an item, a lot of classic Unburden strategys have risen again such as eating a berry or using a Flying Gem.
Whatever the approach, Hawlucha is a powerful and easy to use high risk, high reward Pokemon to use. When not relying on losing an item to power up your attacks and missing with Hi Jump Kick, Fighting/Flying is a very solid type combo that affords Hawlucha good coverage and decent resists at the expense of classic Stealth Rocks weakness. Still, you don't need to put a lot of thought into using Hawlucha effectively. Just give him an item he can afford to lose and start spamming attack until everything is dead or it is dead. He's a hawk-luchador for crying out loud. Stop thinking about it and start kicking things into dust!
You may remember Togekiss from Diamond/Pearl. This fat little dove was introduced as a new evolution to Togepi as a way to make the Togepi line more powerful. Before X and Y, it was a normal/flying like a lot of flying Pokemon. With fairy Pokemon all the rage now though, Togekiss feels like a brand new Pokemon as a fairy/flying Pokemon.
This means in addition to being immune to dragons, Togekiss now resists dark attacks as well as fighting by a whopping 4x. And the release of a new game is a good time to be around, since Togekiss' new weaknesses, poison and steel, are still rarely seen in use. People are still in the mentality of ignoring poison and steel attacks for coverage against the new fairy-type so all Togekiss has to worry are ice, electric, and rock attacks. But with Togekiss' robust special defense, it can shrug off a lot of powerful attacks.
Togekiss' own repertoire of attacks hasn't changed much, but simply changing its typing has given it a huge buff.
Blaziken's hidden ability has always been the coveted Speed Boost, something which increases its speed with every passing turn. It's always been locked away but finally, not only has it been released over mystery gift, Blaziken gets a mega evolution that raises its attack to ridiculous, overkill levels. A well raised Blaziken that mega evolves at level 50 could very well hit an attack of 200!
Together with Speed Boost increasing speed, one turn of Swords Dance boosting attack, and something like Hi Jump Kick, you have a threat that you have to shut down immediately because once the snowball gets going with Mega Blaziken, there's very little you can do to stop it.
While the classic fire/fighting combo gets walled by fairy-types, Blaziken can easily learn Poison Jab to take care of any pesky problems. Once fairies have been dealt with, its hard to think of anything that could possibly survive a single hit from this monster.
It seems ghost Pokemon always seem to have the best design and Trevenant, a grass/ghost Pokemon, is another example of great ghost Pokemon.
It just so happens that grass/ghost can be used as a decent defense in typical grass stall shenanigans i.e. leech seed sets. Trevenant also has the decent bulk though not stellar. Most of its success in stall comes from a couple of great resistances from its grass typing as well as access to leech seed. It also has two great abilities to choose from: Natural Cure and Harvest. While Pokemon like Starmie have Natural Cure as well, encouraging you to absorb statuses or even Rest to switch out, Trevenant is one of the better Pokemon to use Harvest. Harvest allows Trevenant to reuse its berry indefinitely, so a Sitrus berry can keep it healthy for a while combined with Leech Seed and leftovers.
So people think a living key ring is a stupid idea for a Pokemon? People won't be thinking its stupid when they get paralyzed by Thunder Wave. Steel/fairy is actually a deceptively great combo to take hits with. But that's not the real draw here. Klefki gets Prankster as an ability and is the first Pokemon to make real great use of it after the likes of Whimsicott from Black and White.
I already mentioned its great typing for taking hits. Its also capable of putting up the dreaded dual screens, Reflect and Light Screen. And thanks to Prankster, non-damaging attacks receive a +1 priority as if they were Quick Attack. Remember when I said a Pokemon like Blaziken might run away with the win? Klefki can get in with its own emergency brakes and paralyze it, stopping its momentum cold in its tracks.
Klefki has annoyingly good potential for being annoying. So long as you have a Klefki on your team ready for revenge, no sweeper is safe to head onto the field.