Well, Balmung and Gilgamesh are far and away the most populated servers with RP on them (Balmung being the biggest of the two), but I've heard of people attempting to start up roleplay on Jenova and other smaller servers (like Faerie).
Unless you're really interested in helping RP blossom on servers where it is practically nonexistent (save for these few individuals and linkshells), I recommend that you wait until just after a server maintenance to try and make your character(s) on your desired server, either Balmung or Gilgamesh. I was able to sneak two characters onto Balmung just after a maintenance about two weeks ago. Though, they tend to be pretty late (or early, depending on your time zone), so they might require some finagling, and they aren't always a surefire of success. I've sat through a couple maintenances where the servers stayed closed even after the servers came back online.
Though, if even that fails to work, you can always server transfer your characters from any server to Balmung/Gilgamesh. And, if memory serves me correctly (though I definitely could be wrong), a single purchase of a server transfer will allow you to move multiple characters from one server to another, regardless of level, story, etc. I'll probably end up doing that somewhere down the line myself. $18.00 is a small price to pay for enjoyable RP on a populated server, in my opinion. Especially considering the fact that I've already put about ~130 hours on this game after only having it for just shy of a month, myself.
It's up to you, really.
TLDR; money solves everything
Unless you're really interested in helping RP blossom on servers where it is practically nonexistent (save for these few individuals and linkshells), I recommend that you wait until just after a server maintenance to try and make your character(s) on your desired server, either Balmung or Gilgamesh. I was able to sneak two characters onto Balmung just after a maintenance about two weeks ago. Though, they tend to be pretty late (or early, depending on your time zone), so they might require some finagling, and they aren't always a surefire of success. I've sat through a couple maintenances where the servers stayed closed even after the servers came back online.
Though, if even that fails to work, you can always server transfer your characters from any server to Balmung/Gilgamesh. And, if memory serves me correctly (though I definitely could be wrong), a single purchase of a server transfer will allow you to move multiple characters from one server to another, regardless of level, story, etc. I'll probably end up doing that somewhere down the line myself. $18.00 is a small price to pay for enjoyable RP on a populated server, in my opinion. Especially considering the fact that I've already put about ~130 hours on this game after only having it for just shy of a month, myself.
It's up to you, really.
TLDR; money solves everything