What is the most populated server?

[Atoli]

Balmung and Gilgamesh for NA,
Ragnarok and Cerberus for EU,
Shinryu and Chocobo for JP.

Since Jenova is a NA server, I assume you want to stay there, so Balmung or Gilgamesh it is.
You should know though, that instances are shared between all servers from one datacenter (aether datacenter in your case).
Balmung and Gilgamesh are on the same datacenter, so transfering will NOT change or improve your queue times in any way,
since you are already playing together with those server's players in instanced content anyway.