DS9 is what you might call the "primary" hub, usually in Quark's.
Roleplayers can also be found at Starfleet Academy, though the Academy event can really get annoying when it's on. There are some of us that will also visit Hathon, Bajor when we need a break from the bar scene. Look for groups of people around the waterfall area.
The IS roleplay to be found on Drozana, but I wouldn't start the search there. At least among the people I talk to, its looked at as the equivalent of Goldshire in WoW.
KDF side, I really don't know much about, I usually see KDF roleplayers at DS9, but I don't what, if any, the active hubs are elsewhere.
Fed side, Vulcan, Andor, and Risa are usually devoid of life. ESD will occassionally see a few people get together, though it doesn't last long before some moron shows up to cause grief. I've seen a couple people on K-7, and I don't believe I ever see anyone on 39-Sierra. YMMV, though, I'm sure I don't know every Fed-side RP group in the game.