Great question BB, I chose sex but as others have stated it's the combination of all and the journey to get there that is the most exciting.
I think "firsts" would be an appropriate category here. Probably the most fun I have on RLC is experiencing a new arrival/couple's personality and relationship, their first nudity, masterbation and sex. Sometimes we get it all in a matter of hours, sometimes it takes years like it the case of K&K before we get to see all they have to offer. Making it that much more exciting when it finally happens.