As Leia said, Gor is popular, as is urban. They are probably closer to equal, but the urban is spread out among many different group, some quite large, where gor is (somewhat) tied together. And depending on how strictly or loosely you define each can change the numbers a bit. For example, instead of gor, saying fantasy BDSM increases that side a bit, but saying Strict gor, or by the books as they call it, and that side shrinks. School RP is somewhat large as well, though a lot of it is PG, which doesn't sit well for certain types.
But the real answer is that nearly any RP is out there somewhere, and looking for the biggest does not mean the best, and it most assuredly does not mean the best for you. You should start by looking for what interests you specifically, rather than what is popular. Once you get to a modest size, good RP can and does happen, and generally doubling or tripling or more that size doesn't really change the quantity of good RP, it often lowers it a bit honestly.
