Most RPers have congregated on the Aether datacenter as said above, but there are groups on Behemoth, Famfrit, and Lamia who are active, I think.

In terms of population, Balmung > Mateus > Gilgamesh / Jenova (afaik, most RPers on Gilgamesh left for Balmung years ago when more raiders started to transfer in. I've seen more Jenova activity than Gilgamesh, but this could just mean they're not using the same resources as me).

Lodestone RP Linkshell search

Lamia groups I'm aware of:
RP Lamia
Arche Koeln