What's the best element?
Fake news. This was lightning theory as put forward by an old-ass man in a time where it was a skill that was limited only to royalty. We see in Korra that normal-ass people can use it predictably and reliably to the point it becomes a job, and Mako specifically can shoot lightning while bloodbent as well as change the voltage his lightning strike - indicating a definite ability to vary electric potential, because that's what voltage is. Iroh might have been a genius during and after The Last Airbender, but he was old and there was a solid 70 years of extremely quick innovation after that time; he's not omnipotent and in some ways is explicitly contradicted by, for example, the ease of use of lightning in the future and the much finer degree of control over it.Now hold on just a diddly darn second. The firebenders were very specifically said not to be able to control lightning. They could only create it, and send it in one direction.
"Remember, once you separate the energy, you do not command it, you are simply its humble guide."
If we're going off Avatar rules, there's no way that fire powers let you control electromagnetic waves.
The speed of a lightning strike vastly exceeds the speed of any fluid in the body - you'd just shock or electrocute the absolute shit out of yourself and it would go through your dick. Which, I mean - if you're into that, I guess.Also remember how the power to redirect lightning was originally based on waterbending? Water conducts electricity. I can just pee on your fire and take your lightning for myself.
All it could do is move in circles, and I gotta be honest - the ability to change the orbital position of a planet, while cool, isn't really that great in general, especially since you'd probably end up killing everyone on it by trying to. Also they considered "Earth" not to be one of those planets because Greek philosophy didn't think of the Earth a celestial body. While we now know that they were wrong, per Greek philosophy having aether is practically fucking useless.As for aether, if we're going off the original meaning that it's the material that controls celestial movements, wouldn't power over aether let you control the motion of the planets themselves? That'd make it so absurdly powerful that it would be on a scale that's completely useless to human existence.
But there's another discarded scientific meaning of aether that fits better with the classic meaning, which is that it's the medium for gravity, in other words, space-time. So if we're using that definition, aether gives you gravity powers, which basically let's you control all the elements anyway, and perhaps even time powers.
Are you sure you wouldn't rather pick aether, Tirin?
On top of this, aether has been pretty convincingly proven not to exist nor to be necessary as a medium for gravity, electromagnetism, or any other fundamental forces. If you're gonna say I can have the powers of aether in the sense of "space in general as a readily-manipulated element" - then sure, I'll take that over any other element what with having literal omnipotence, and also submit that it's easily the best for that reason.That theory was advanced literally millennia after "aether" was proposed as a classical element and has no bearing on either the classical analysis of the elements nor on "how it works" in Avatar, which is pretty clearly "fire gets electric (and thus magnetic) fields".
Aether sucks, fire fuckin' rules.