RE: The Best Ex
Title should be Best EX, not Ex.
Anyway...
Mewtwo probally takes the prize for this one, being able to fit into any deck and deal 80+ damage easily. And even more in decks that have energy excel, though even in the ones that dont, it can up its damage output easily with DCE.
Darkrai however makes a strong second contender as it can give free retreat to any pokemon that has a Blend (GPFD)/Prism/Dark energy and also has a great attack, plus resistance to Mewtwo.
Landorus seems over hyped to me, though I cant deny that it is a great card. Though I doubt it will be as much as a staple as people make it seem.
Keldeo...haha, very funny. How can this be any good with Mewtwo in our format? It takes too many energy to do any decent ammount of damage and Mewtwo can easily OHKO it. Yes, it has a good ability, but if it's being OHKOd anyway, what does it matter?
Raikou is awesome, it has free retreat in all Eel decks that use Skyarrow Bridge, and what Eels deck doesnt? It can hit 100 on any of your opponet's pokemon (aside from Squirtle) and then discards all it's energy, which is good for Eel's ability and cancles the extra damage Mewtwo would other wise do.
Raquaza may not be as good as Raikou or Mewtwo, but it can hit for outrageous ammounts of damage with very little energy. Though the fact that it needs Fire or Prism energy in Eels decks (or Lightning energy in Emboar decks...nvm, thats suicide) kinda hinders it's preformance.
Groudon is pretty good imo, being able to spread after T2 and OHKOing with it's second attack half the time. Though right now, with Keldeo being so hyped, Water weakness isnt very good and such a bulky EX. Though it does see enough play in Terrakion and Klinklang (dont laugh, it works) decks.
Terrakion is the only energy excel. we have for Fighting types, but in addition to that it can hit for 90 damage with 3 energy. Kinda slow, but can OHKO Darkrai (without Eviolite) and 2HKO everything else. But because it takes at least 3 turns to set up, it does make it difficult to keep them coming. However in Terrakion/Darkrai decks, it can hit on T2 with Dark Patch and Energy Switch.
Registeel is seen often as a tech in Eels, I guess because of it's resistance to Mewtwo and the fact it can 2HKO multiple opposing Deinos and Tynamos in 2 turns. And in Klinklang it really is difficult to KO because after it uses Protect Charge, which reduces damage to it by 20, and an attached Evoilite.
Regigigas may see a come back with the Apserita City Gym stadium card, which then gives it 200 HP, which can be boosted even further with Giant Cape or Eviolite. But who can tell for sure?
Tornadus is one of the faster EXs in our format, somewhere up there with Darkrai and Mewtwo, because it can hit for 60 T1 more often than not, with just a stadium (again, Asperita City Gym helps in this case) and a DCE. Then can hit for 100 T3 for just abother energy, and even though it soemtimes need to discard and energy, it doesnt hurt Darkrai or Eels decks. It's also a great tech in those decks since it has resistance to Fighting which both Darkrai and most Eels attackers are weak to.
Kyogre is only really good as a tech in Keldeo or Klinklang, but can hit benched pokemon for a decent ammount of damage so you can finish them off later.
Shaymin is fairly frail (lol) with only 110 HP, but when used wisely and at the right time, can make or break the game. It can hit for enough damage to get a final KO with only 2 energy. It's especially useful in Darkrai decks for the last KO on a Terrakion/Groudon/Landorus.
Ho-Oh is a speedy and interesting EX, because of it's ability. Ho-Oh decks use TONS (if not all) of different kinds of energy. And can usually hit between T1 and T3 depending on how lucky you are with coin flips.
Cresselia is only really good as a tech in Darkrai/Hydriegon decks, though it does work if used with the right timing. It's Ability makes it difficult to KO and it's attack's effect prevents it from being revenge killed by Mewtwo or another Cresselia. However, it only works in Darkrai/Hydriegon decks because it can use Blends and then be brought up to the front whenever it wants because you can use Hydriegon's ability to give it 4 energy easily.
Other than that, there's not much else worth mentioning. Mew EX is just bad, Zekrom is out classed by others, ect.