Do you want an actual RP guild or just a “casual and social” one?
If it’s the latter, do pick Argent Dawn. They have enough population to also have a set of non-RP guilds.
Defias Brotherhood cluster is the second biggest RP realm cluster, but people there are more focused on RP. As I reckon, it’s harder to find guilds that do proper content regularly.
Moonglade cluster is currently the smallest, and it has a subset of dedicated RP guilds and a pretty closed set of players who know each other for the most part. It has a few social and proper content guilds, because half the population finds the “RP” part in the realm name optional.