Well I think a little bit different about Donphan than iisnumber though i don't say he's wrong.
The fact is, that the 60 damage for one energy is still pretty good, Donphan's Poke-Body is still pretty nice and the typing is just pretty good for the moment.
Okay the first thing: His attacks
Earthquakes 60 for on {F} still is good, though 60 might sound like nothing. But you shouldn't forget, that there are still many things being KOed by an Earthquake.
Some viable Examples:
Tynamo, Eelectrik, Thundurus, Floink, Magnemite, Magneton, Fliptini, Celebi Prime, Mew Prime, Rotom and most other viable Evolution Basics.
Zekrom and Magnezone are OHKOed with a PlusPower, Regigigas EX and Zekrom EX all are 2HKOed.
Not to shabby right?
His second attack isn't that great, but it's nice for bigger opponents.
So what could be really difficult for Donphan?
Examples:
Kyurem (only sometimes), Kyurem EX both Reshirams, Tornadus, Zapdos and probably Mewtwo EX... and thats it pretty much...
So what to do against them?
Kyurem and Kyurem EX shouldn't be the biggest problems, the Reshirams could be taken out by teching Kyruem, though they also shouldn't see that much play.
Tornadus and Zapdos are difficult because of their resistance, so Ruins of Alph could handle these and last but not least Mewtwo EX.
Mewtwo EX is awesome no doubt, and it is important to have an answer to it.
Well in my opinion Donphan could be the answer itself. Thanks to it's low attacking costs, Mewtwo doesn't get much support for X-Ball from Donphans energies.
IF Donphan can hit first, it could load enough damage on Mewtwo to get a 1-2 prize exchange. If this is not the case, there are always some tricks for Donphan to use.
1. use Max Potion
Max Potion can heal all the damage from Donphan and then just get the energy back through manual attachement. That means, that you probably could do enough damage to KO Mewtwo after he Koed your Donphan.
2. Rocky Helmet
Rocky Helmet gives some extra damage to Mewtwo (or other annoying opponents) that means, that you could load the damage even faster.
3.play Mewtwo EX
Do I have to describe it? I don't think so...
So it has a decent attack (especially with Catchers) a poke Body that let is live long, massive 120 HP (excellent for Stage 1) and a weakness that isn't that playable right now.
Additionally it has a nice Lightning Resistance which is nice in a format with Lightning being so powerful.
It's retreat of four isn't that nice, but wiht a high switch count and the use of Heavy Ball you could use it for your advantage.
The last point is a direct comparison of Donphan and Terrakion NV
1. Stats:
In my opinion: point for Donphan
It has 10 less HP but an auto resistance for 20 against everything.
It has also a {L} resistance while Terrakion has none, the weaknesses are atm both pretty good, because both types only see little play.
Retreat is the same and type is the same too, so i think the advantages are on Donphans side.
2. Attacks:
Donphan:
60 for one {F} with a bench damage for all of your Pokemon and 90 for 3 {F}.
Terrakion:
30 for {F}{C} plus 60 IF one of your pokemon was KOed the round before which is cool, but has a condition that costs you a prize... the second attack does 90 for {F}{F}{C} which is nearly the same as Donphan
I think this is a draw between them because while Donphan does the more consisten damage, Terrakion can be a lot better in some situations
3. The rest:
Terrakion has the advantage of being Basic which means a lot of support as well as that it is better techable and better searchable. These are big bonuses so an extra point for Terrakion here.
--> 2:2 between them
So it isn't the truth that Terrakion ALWAYS outclasses Donphan. They are just pretty different, so that I think both could be the best for different situations.
Donphan is the more consistent Main Attacker, while Terrakion is the awesome guy that can be teched in nearly every deck. I think that Donphan IS still playable, at least as Tier 2 deck or something like that, because it can counter the metagame and probably surprise some people.
P.S.
Don't forget, these are MY opinions, so if you think different I have no problems. If there are grammar or spelling mistakes please don't kill me, it's pretty late here in Europe and I'm hell tired =D