Well, one was actually a short while back, with a friend. Storywise it was an absolute mess; it started off loosely based on supernatural, but eventually we just mixed and matched all kinds of fantasy lore and basically; it was just so much fun.
My rp partner also had a lovely writing style and very original characters, which helped a lot of course. But, beside that I don't even know why I loved it so much, I just did ^_^

Currently I'm in another one and I have to say; I haven't been this insanely motivated to post since that other one. It's basically about two guys who live in Manhattan and their ups and downs, but somehow it has managed to grab me more than any RP in the last while has. 
It's also one of the few RP's that actually really made me cry. Only about three others have managed that, including the one mentioned above :') Not that that is saying much, since I'm emotional as fuck when it comes to fiction, but not many RP's have gotten me to the point where I REALLY needed a kleenex.

I think that currently, those two are my favorites ^_^