In the housing part of the 2.3 patch notes they divided servers in three groups, probably based on active population.

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Group A:

Aegis / Bahamut / Balmung / Behemoth / Cactuar / Carbuncle / Chocobo / Diabolos / Durandal / Excalibur / Fenrir / Garuda / Gilgamesh / Gungnir / Hyperion / Ifrit / Leviathan / Mandragora / Masamune / Midgardsormr / Odin / Ragnarok / Ramuh / Ridill / Sargatanas / Shiva / Siren / Tiamat / Titan / Tonberry / Ultros

Group B

Adamantoise / Alexander / Anima / Atomos / Cerberus / Coeurl / Exodus / Faerie / Goblin / Hades / Ixion / Kujata / Lamia / Malboro / Moogle / Pandaemonium / Phoenix / Typhon / Ultima / Valefor

Group C

Asura / Belias / Brynhildr / Famfrit / Jenova / Lich / Mateus / Shinryu / Unicorn / Yojimbo / Zalera / Zeromus / Zodiark

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My suggestion? Go for a group A server, except for legacy servers and 2-3 more most group A servers aren't that crowded, but will have enough people to fill PF during most of the day.