The most populated are those with X marks which mean transfer or character creations are restricted.
I used to play in Ragnarok and I would say it has solid community, so I would recommend that place..

■European Data Center
▼Preferred Worlds
　None

▼New Worlds
　○ Louisoix

▼Standard Worlds
　○ Cerberus
　○ Lich
　× Moogle
　× Odin
　○ Omega
　× Phoenix
　○ Ragnarok
　× Shiva
　○ Zodiark