The portal to Atlantis is in the South Pole map-- It's hidden up in the Northeast corner of the map. :3 And from there you can find the portal to The Last Cave... But it's really hard to find, I forget where the portal is there, haha.
I'm gonna have to say the best spot for roleplaying would proooobably be Flourite Plains, if only because it's the largest map, has a few different terrain types for folks who wanna RP in different areas, and it's where most people looking for long-term literate RP hang out.