Concept is honestly not #1 on my list. The people playing there and is it run well and fairly is more important to me than the genre. I could probably get into any RP if the people were great.
I agree with Niiigel. I've only ever played in dark urban/post apocalyptic places though because most medieval sims I've visited, I couldn't get into :/
I'm a big fan of Post Apoc, and Cyberpunk. I used to like modern supernatural well enough, but it kinda got old for me, mostly because it just seemed like the same story rehashed over and over without anything
I used to un a dark fantasy/post apoc kinda place and even in a gorean one (in the first gorean sim in SL) im like Nigel, if the RP is great i could get into it.
What i liked from both those places was that they were giving lessons and classes on how to do proper RP, for me was amazing. It helped me improve my english and i learned how to RP in a para-RP.
My favorite tends to be historical RP, but not medieval RP. Like, Roman times, Victorian, Edo era. Other than that, you know the things. YOU KNOW ALL THE THINGS.