I'd like to think it was my SilverValley roleplay.
Even though it's not nearly as good as it was, in its golden months we created some amazing stories, and for that I give a lot of credit to Sir Dooble, as without him many of my RPs are in shambles.
:)
Also, another that I'm fairly proud of, even though, now looking back, it's not as impressively built as it could have been, is my
Shattered Memories: A Space Roleplay, which was less of an accomplishment in terms of building, but more in the fact that I had a lot of people having a lot of fun there.
And it was fun to build, even if most of it was duped, and the original wasn't intended to be a spacestation.
Unfortuantely, I have lost this build, alongside many of my most favourites. Too many computer changes and memory wipes have occured, really.