My tier list of EX cards, the tier list I choose was based off of a none break through format, though there is always a ton of options for it in the format.
Tyrnatrum EX: tier 1, the fact that it's base attack mixed with faded town and muscle band should not be under looked letting this card hit a max dmg out put of 230 against mega decks, it's also all the deck support for this card pushes it up to tier 1.
Seismetoad EX: tier 2, initial tier 3. Grass has become to much of a viable threat in standard which lowers seismetoad's viability but at the same time offer's options and is effective against some decks.
Manectric EX: tier 1-2, This is one of those EX's that does do a lot in our format, with the right combo can win almost all of it's initial match ups but also has hard match ups due to it's popularity turning the deck into a target overall, however there is so many good cards in standard to be paried with manectric it's more about finding that perfect partner.
Mega Rayquaza (colourless) tier 1, Don't let it's lightning weakness fool you, this happens to be one of the fastest and overall consistent dmg dealing deck in standard, being able to have access to setup and deal crazy amounts of dmg in next to no time at all, certainly a u/u deck but still a dangerous one.
Yveltal EX: tier 2, initial tier 3, The best way to explain this is lack of viable support and good matchups.
Mega Sceptile EX: Tier 2, There is solid support for the deck, not a whole lot of support options, Grass is prominent so is it's weakness.
Mega Tyrnaitar EX: rogue, it's a fun card to play, it just doesn't have enough testing to give it a name for itself.
Mega Ampharos EX: tier 2-3, this is another fun deck, though it has deck support, setup options and more on the way it is a hard deck to ensure in standard atm, after break through it could be a different story.
Mega Charizard X or Y or any EX: rogue, Though there is some support for fire pokemon the EX's seem to hold a lack luster stance to they're non-EX counter parts which offer more setup options overall, maybe after break through but the limited 190 attack base against megas leaves alot of they're attacks to be undesirable.
Lucario EX: tier 2, this card has lot's of support options and high dmg out put modifier's mostly in standard a U/U card but still a viable attacker in the format.
Dragonite EX: tier 2, funny thing about this card is you can tech it in mega manectric or sceptile and take glorious ko's on mega's, shaymin's or otherwise, adding muscle band an a dragonite is fairly viable when you think about it.
Aegislash EX tier 1, the ability, attack and the deck support the card has offer multiple options of play, people may play a hex maniac, but they will play more special energy in their deck overall making this one of the hardest cards to out play in the format.
Dialgia EX tier 2-3, the only issue for this card is needing the attack to proc the wall and the fact that it only wall's EX's.
Malamar EX tier 1-2, the ability and attack leaves the deck with much to be desired, however the card doesn't seem to have alot of deck options and is more of a tech.
camelrupt EX rogue, playing this card is living the dream, when all the options for a card can fit into one deck it's hard to deal with various matchups
Wailord EX tier 1, indisputably one of the best stall function based cards in the format, it's 250 hp makes it so, it's grass weakness is a little left undesirable but weakness policy is a good friend.
Primal Kyogre EX, tier 2, Solid attack, has alot of strong EX matchups but weak matchups in the same token, solid support and deck options
Primal groudon EX, tier 1-2, the deck options and support make this deck so high up on the scale, making cards like shaymin and hoopa junk cards and unfavorable matchup's favorable.
Mega Aggron EX, tier 3, though it has alot of deck's that beat it, it's dmg output, typing and various options leave it to be very desirable, this card could see a jump and win a lot in standard if something come's in and offer's it something.
Trevenant EX, tier 2, sceptile tech attacker which would get around corner's for sceptile.
Mega gardevior EX, rogue, fairy doesn't offer alot outside of toolbox, steel as it's biggest match-up threatens the format to much for fairies to thrive, if fairies got more support maybe.
Mega Gallade EX, rogue, I've only seen this deck to well once, though gallade has lot's of options in the format it's typing take's a hit, especially once breakthrough comes out.
Mega latios EX, rogue, The day after the pre release I got 4 mega latios and started testing, imo there was so many support options that make the deck solid in various matchups, however it's speed isn't that consistent making it rather hard to play the deck competitively.
Mega Rayquaza EX (dragon) tier 2-3, I have seen many different ways of playing the deck, all of which has been fairly effective, the issue with the deck has been consistency.
Machamp EX, tier 2-3, Machamp/ariados seem's to be the best combo for the EX, however due to the amount of speed the deck is allotted to and then typing it has some fairly bad matchups at the same times strong ones.
Giratina EX, tier 1, 2 things about this card make it far to good to play, 1. it's ability and attack make some matchups almost impossible to win, 2. It has so many viable and strong deck options that make it scary good in both standard and expanded.
Lugia EX, tier 2-3, attack is good, typing is bad and good, colourless means free reign in almost every deck but at the same time risky to play in some matchups
Again this is my opinion, of course things change depending on set and testing. But why leave out hoopa and shaymin? Why do I have to talk about a card pretty much everyone plays in pretty much every deck they run.