I have an alt in Balmung and I can tell you that's it's VERY large and open RP is common throughout there are also occasional server events that RPers like to plan and it's available for everyone if interested: Masquerade Balls, Parades, Fight Tournaments, etc.

The 2nd in terms of RP population is Gilgamesh. It used to be more popular, but it's dwindled down and I think many RPers went to Balmung. There's RP, but it's more private and limited to FCs, LSs and rooms. You pretty much have to work to find RPers.

Other servers ... I don't know, but when it comes to RP servers it's 1st: Balmung and 2nd Gilgamesh