Which world is the best...

[HyoMinPark]

Odd... For me, siren, gilgamesh, midgardsormr, adamantoise, and cactaur are all locked. If these servers are really locked right now, your choices are narrowed down to jenova, faerie, and sargatanas. You can also try waiting for cactuar to open up.

Cactuar and Gilgamesh are currently congested and not open to new characters or transfers because there’s too many people already on the server; where as the others are just locked to character creation right now (but not transfers) due to the amount of people currently logged in. So the OP could always transfer to any of the Aether servers outside of Cactuar and Gilgamesh regardless of them currently being locked to creating new characters.

The earliest Cactuar would open would be Early Access/ShB Launch, but given how some unofficial censes are calling it the 4th most populated server (and stating that it had the highest influx of characters since the Crystal/Aether split), I wouldn’t be surprised to see it remain congested.

[Senn]

...

Oh I see. So would the most populated servers be balmung, maetus, gilgamesh, and cactuar in that order?

[HyoMinPark]

Oh I see. So would the most populated servers be balmung, maetus, gilgamesh, and cactuar in that order?

Yes, that is the current order according to LuckyBancho’s latest census.

To the OP: If you want to hang out with friends, while you won’t be able to join their FC, you can still World Visit to Cactuar and hang out with them there, as well as do cross-realm PFs for content and stuff. So any of the Aether servers would do. From what I know of them—

Gilgamesh & Sargatanas — raid servers
Cactuar — sort of an “everything” server (probably why it exploded in popularity—we used to be lower mid-pop in HW)
Faerie — I’ve heard they have a very active LGBT community there
Siren — I think they’re a bit of everything; always heard they were kind of a “midcore, chill” server
Midgardsormr — same as Siren from everything I’ve heard
Jenova — I think there were a lot of people from Balmung that moved to Jenova. I’m not sure what kind of community they have, but I have friends on there that are friendly and like to help people

[linay]

Oh I see. So would the most populated servers be balmung, maetus, gilgamesh, and cactuar in that order?

If so, I'm surprised that Mateus is not a congested server.