Every deck huh. Empoleon, Mono-fighting, skittles, accelgor gothitelle. Theres 4 decks that no matter how hard you try cant 1 shot rayquaza EX.EVERY deck can tech in mewtwos, NO deck can just tech in a dragon to deal with rayquaza ex. That,in my opinion, makes Mewtwo THE ULTIMATE GLASS CANNON.
I think you are exaggerating.
Mewtwo EX as the "ultimate" glass cannon? The idea behind a "glass cannon" is a strong attacker that is easy for the opposition to take out next turn,
not a strong attacker that gets taken out by itself. When
Mewtwo EX is everywhere and if not a deck's main attackers one of it's secondary attackers, then it
functions similarly to a glass cannon due to extenuating circumstances... just as
Rayquaza EX would if circumstances made
Rayquaza EX decks the only viable choice for Organized Play.
Yes, the fact that
Mewtwo EX can be used in almost any deck to good effect is far less specialized than a format where you run
Rayquaza EX or go home, or less bizarre run a "big, fast Dragon" or go home, but a glass cannon is more something that any remotely competent attacker can take out in one shot.
Victini (
BW: Noble Victories 15/101) or
Victini (
BW: Noble Victories 43/101) are much better examples of "glass cannons", since they hit hard for little Energy but their HP is below the going average per-turn damage for competitive decks.
An even better example, but sadly not from Pokémon, was a card from Yu-Gi-Oh (at least while I played that game) called
Goblin Attack Force. Yu-Gi-Oh mechanics are different than Pokémon, but as a crude approximation, it had 2300 ATK as a Level 4 Monster but 0 DEF, and shifted to DEF position after attacking (and wouldn't let you change position manually the next turn). It would be like getting a 130 HP
Terrakion that hit for 120 damage for
fighting
but placed 12 damage counters on itself at the same time.