As every one probably knows by now, ascendance is probably the best cooldown in game. I feel however each one is better than another.
For starters I play an Elemental shaman. My fav spec since wrath. And being able to spam Lava Burst was the best thing that could happen.
My favorite Ascendance too by far, but if I had to rank which spec was the best for Ascendance I'd say this:
Number one reason enhance is the best is because if you have no cc, you can't counter it, and its damage is just unreal even if your heals is free, you can die.
For Ele its one thing to say its underpowered, because its not at all, but anothing to say is over powered, Ele's ascendance is likely the biggest game changer of the three if you can get the full duration out bexause you can force almost all defensives with it. However it is ciu terable and with the state of warriors, if they sit you, chance is you are only gonna get a few casts out, which you could juat proc anyways.
Restoration has the lighter of the two and usually the last oh shit button a shaman pushes. The biggest downdownfall of it is that it splits your heals making it not nearly as potent as say wings or tree for example. The 100% to all heals can be usefull of no other team mate is verylow on heals.
Unlike most cooldowns in the game, ascendance while super noticeable isn't that noticeable "on use" except for enhancement which boost not only a thunderstorm but dark blue clouds, thus you can immediatly push cc on him.
For ele is a faint red smoke which probably makes it the least noticeable of the three.
Last for resto, just a bunch of water goes everywhere, the coolest looking in my opinion of the three, and very easy to capture when its used.
Some adjustments made to all 3 could be, tone the damage down on enhancement, remove the aoe healing from the resto, ad have the elemental force lightning bolt do 50% more damage or be a guaranteed crit.