The first room I decorated was the Mad Hatter's Tea Party. It was absolutely wonderful to me, since I'm a HUGE Alice in Wonderland fan and AIW is often forgotten when it comes to merchandise.
I had several rooms right when I started, though. Since I created my character in the park. The second of those I went to decorate was the Tiki Room since I love the show at DL and I went to school in Hawaii for a while.
Those are both still two of my favorite rooms.