However, I don't really think "the best RP" can be defined. There have been some truly excellent moments over the last however long I've been playing here. Some wonderful story based things, and some fantastic bits of acting in game. But I don't think I'd be able to pin down where "the best" RP has come from overall.
The only defining factor would be that it comes from people who have developed the involved characters to the point where they know intuitively how the character thinks and feels in any given situation, and allow themselves to be that character for the amount of time they're roleplaying it.