Pokemon: Xerneas
Moves: Geomancy, Thunderbolt/Flash Cannon, Moonblast, Psystrike
Item: Power Herb
Pokemon: Aron
Level: 1
Moves: Endeavor, Protect, Sleep Talk, Toxic
Item: Shell Bell
Pokemon: Cloyster
Moves: Shell Smash, Icicle Spear, Rock Throw, etc
Item: Focus Sash
of all of these, I think Xerneas is the most threatening to most teams as after it's Geomancy'd in one turn thanks to the power herb, can proceed to one-shot all other X&Y Available legendary pokemon, devastate common stallers such as Chancey and is able to slowly chip away at shuckle even with it's constant rests. Xerneas' only true opponent is Scizor, as it's still the only pokemon that can outspeed Xerneas' 2x speed stat boost with bullet punch and do a 1-hit KO. Aegislash is another potential opponent with iron head, however Xerneas is typically able to land it's Thunderbolt first and takes it's health down to yellow, and if iron head fails to 1-shot Xerneas...
Need proof of Xerneas' OP?
Check out my 6 vs 1 Battle with your Vs. Recorder: HKGG-WWWW-WWW3-78SM
P.S. this battle is prior to my resetting and redoing Xerneas' EVs for speed and special attack. At the time of this battle, Xerneas' EVs were still default from using Xerneas during the normal gameplay story.