Ryzom

Samsara, Red Ribbon Army, Order of the Dragonblades are the most RP focused established guilds at the moment. But do consider new guilds as well, you wont get as much freebies but instead be useful in the guild. Karavaneer Desert Legion sounds to me like a guild with lots of RP potential, I'm really tempted to make an alt so I can join the legion.

"Semi-RP" (I don't like the definition of that) guilds will also throw you into the spectrum of politics on Atys and let you roleplay your character.