IMO, these are the 5 most broken cards in this set (from 5 to 1):
5. Tyranitar: Play this card with Darkrai LV.X and Weavile SW = Amazing. It make take a couple of turns, but 80 to each of your opponent's pokemon is just groundbreaking.
4. Raichu LV.X: Attack twice in 1 turn? This card is ringing the word broken. Of course you have to use Voltage Shoot once, but you get to choose the next attack. With so many combinations with the new raichu, the md raichu, and the mt raichu, endless combinations can be made.
3. Regigigas + Regigigas LV.X: These cards are very, very interesting. Interestingly broken. Let's see: Get Regigigas and 3 regis out, Level up, giga blaster, then giga power next turn, then giga blaster, etc. Sacrifice at the right time on Regigigas, and it can be an interesting game.
2. Gengar: It's just incredible. The Poke-Power is a bit flippy (Love the name!), but flip a heads and it brings payback. The first attack brings even more envy. For one energy, you can place up to 6 damage counters on one of your opponent's pokemon. The second attack also helps with Lumineon, as it gets to look at your opponent's hand beforehand so you can get a strategy of what to do.
1. Salamence: I do think this card is the most broken in the set. Just about every deck plays a pokemon with 120 + HP. So T2 you can either knock it out or leave a lot of damage on it. Also 2 energy for 50 isn't bad either.
Once again, this is just my opinion for the 5 most broken cards in the set.
6. Sableye
7. Luimineon
8. Shaymin X
9. Duskinoir X
10. Torterra