Maybe Aegislash can be Uber, you know? It would be the new "Excadrill"!
I really hope you're joking, haha. It's too predictable with King's Shield and has a case of 4MSS no matter what it tries to do. Sure, it can demolish any counter with the right move, but so can a lot of Pokemon (Jirachi, Keldeo, Breloom, etc.). The fact is, Aegislash doesn't have 60/150/150/150/150/60 stats, and if you can predict around it then it's pretty easy to get rid of. It's still a threat though, and the most effective sets are arguably those that don't even use King's Shield.
My ideas for the top threats of XY, including new Mega Evolutions:
Go straight to Ubers:
1. Xerneas
2. Mega Gengar
3. Fairy Arceus
4. Yveltal
1. Talonflame - Yes it's walled by Rock-types. Run a Dugtrio to get rid of them. Afterwards it can sweep. Rotom-W needs defensive investment, as the current "bulky attacker" set at 232/0 Def is OHKOed by a +2 Sky Plate Brave Bird after SR. Heatran is not only molested by Dugtrio, but has to use Ancientpower to do any relevant damage to Talonflame. Talonflame also can afford to run as many as 192 HP EVs--it only needs 60 Speed to outspeed Jolly Excadrill to hit it with Flare Blitz, as everything faster either dies to Brave Bird or doesn't do much damage to Talonflame (although you might want to run more EVs to deal with Jirachi.) It also happens to 2HKO Barbaracle at +2 with a Sky Plate Brave Bird, assuming lack of defensive investment on the former, which means it's not exactly a safe switch-in.
2. Mega Lucario - It can do so many things, like Swords Dance and Nasty Plot, and the fact is that you don't know what it's running until it sets up, which is similar to Mew and Rayquaza in their respective tiers. A +2 Close Combat hits really hard, destroying things like Skarmory easily, and even having a 40% chance to 2HKO 252/252 Impish Gliscor after SR. Crunch lets it deal with Aegislash with proper prediction, and Bullet Punch kills a lot of faster things, like even Mega Gengar at +2.
3. Aegislash - The hype will eventually settle down. Aegislash has 4MSS, and although it can run physical, special, or mixed sets, those sets all have plenty of counters that make life very difficult for Aegislash. Once you know what it is running, it is a straightforward process to beat it. The trouble is telling the sets apart, and since Swords Dance is the most common and most powerful, predicting which set it's running isn't that bad even if you get it wrong. Also mad props to this thing-it is one of two Pokemon to reliably check Xerneas, which will give it Ubers usage.
4. Zygarde - It's the only Dragon-type to get Coil. It also gets Extremespeed and EdgeQuake coverage. Too bad it doesn't get Dragon Rush (seriously?). But regardless, Zygarde is still a good bulky set-up sweeper. Just watch out for stuff that can survive a boosted Extremespeed and outspeed and OHKO, or survive a boosted Edge/Quake and OHKO.
5. Greninja - It's the only viable user of Protean (lol Kecleon), which is nice. It gets Spikes, and gets a bunch of coverage moves. As it stands, Protean requires quite a bit of prediction, and if you get something wrong you could lose your Greninja. If only it could Rapid Spin...
6. Mega Blastoise: A Spinner that can defeat all Spinblockers? I'll take it. Even if it takes up your Mega slot, if your team isn't the type that uses Mega Lucario or Mega Mawile (or Mega Gengar before that inevitably gets banned), it's well worth it.
Honorable mentions:
Mega Mawile: 678 Attack and Swords Dance, and you get to Intimidate the foe before Mega Evolving. Also, it has the best defensive typing in existence. Too bad it has such bad speed, although it can use Sucker Punch very well.
Noviern: Attacks go through Subs, gets Draco Meteor and Boomburst, and has a decent defensive typing which lets it switch in on lots of moves. It'll be right up there with Crobat as revenge killers this gen.
Sylveon: This thing defines "defensive Fairy-type", and even has a very usable Special Attack stat and access to Pixilate Hyper Voice (post-Pokebank) to boot. It'll definitely be one of the premier Fairy-type walls.
Florges: Operates the same way as Sylveon, except it has a little higher special bulk and a little lower physical bulk, as well as less of a movepool. It will probably end up in a tier close to Sylveon.