Mega Gengar ex (Phantom Gate [XY4] 097/088) (PSA 10 Price: ~US$697) Although there were two Mega Gengar ex cards released, theres a very obvious reason why this Ultra Rare (UR) card is so much more valuable than the other card from the XY-P Promotional Cards
You need three energy to use this ability, but that's less than some of the best Pokmon TCG Pocket moves - in particular Mewtwo Ex's or Charizard Ex's devastating attacks
Upgrades Unleashed: Theme:Modify(Auras, Equipment, counters)
In addition to dropping Mokey Mokey with Future Fusion, it's pretty good if you can get 3 of his Mokey Mokeys from here and throw in various cards
A perfect place to start if you want to play competitively
There are two higher scoring legendary, but they are more for the 99, so this is the highest for Command Zone purposes