This deck is certainly one of the more interesting, diverse, and powerful decks out there. It can run a variety of techs but only a few are truly good.
1. Shaymin EX- Already been discussed here, not to mention it swings the Keldeo matchup.
2. Sigilyph- With most builds of the deck already running Eviolite, this guy can be included as a legitimate tank. All Keldeo builds not running Pony 47 can KO it with Blastoise, but then you can just Dragonblast it. Then, they just lost 5 Energy AND their accelerator.
3. Mewtwo EX- obvious synergy with the deck, as well as being overly awesome anyway.
4. Keldeo EX- Great for countering Landorus, and Prism is okay in this deck too. Also gets you out of status and random annoying effects.
5. Giratina EX- Great counter to Sigilyph and one of the tankiest ones out there. Shredding to 2HKO'ing everything can work and possibly better than Night Spear because of the almost nonexistent Weakness.
6 Cresselia EX- I personally hate this guy but people seem to have success with this.
These weren't in order; if they were then they would all tie for first and Cresselia would be 2nd
Honoable mentions- Roserade DRX Ability- can actually work in here I've tried it. If you can make room, you can also play it with Devolution Spray which is better if you read it as: "Next Turn, search your deck for any card" and is Junk Huntable.
-Revive Like Dark Void said it is great in here sometimes XP.
Failures:
These are some cards that would seem good in this deck but are really not.
1. Virizion EPO- Can counter Keldeo but takes three Energy with a frail 100 HP and two have to be Blends. If an Eviolite is not equipped then a Mewtwo can just revenge with two Energy putting you in a less-than-desireable spot.
2. Registeel EX- Cannot have enough of an impact on a game and you beat Zeels anyway. It could help against RayEels but that is not popular enough to warrant playing this. Also, Squirtle's Ability blocks it.
3. Meloetta- Wastes a Blend and is super frail. Also, you can counter Sigilyph or Mewtwo easier with other Pokemon.