So....saturday won, as expected and i thanx all the ponies who voted and offered polls. But this weeks will be...IF YOU COULD HAVE ONE RULER OF EQUESTRIA, WHO WOULD IT BE thats right, 1 enjoy!!

520146 One rule? Who would it be? You might want to take a second look at your spelling.

520275 wow, thanx man.....i am sooo illiterite

524619 Ah, sorry.

Anyway, one ruler? I'd probably go with Cadence.

...yeah, you read that right.
The problem is that Luna and Celestia, while no doubt competent on their own, do rely on the other, emotionally, to a certain extent. They're like Yin and Yang, you can't have one without the other, and while it isn't really outright stated, it is subtly implied that Celestia felt pretty bad about the whole banishment schtick.
Cadence is the only other (canon) alicorn, so she would be able to handle the sun-moon cycle in their stead (assuming they didn't just step down from their positions for whatever reason to allow this scenario and would keep doing the sun-moon thing) and she seems fairly well adjusted, emotionally speaking, while strong enough to handle any outside threats.
(I gathered from the Crystal Empire episode that she probably could have handled Sombra on her own, but he was still being elusive and their attention was more on figuring out what was with the empire. By the time the mane six arrived, she wasn't exactly at her strongest. Similarly, Chrysalis probably ambushed, incapacitated, and imprisoned her before she could react.)