I know I have one, I'd bet that most of us do. What's that one thing you have always wanted to build? Maybe it was at some level the very reason you got interested in woodworking in the first place. You say to yourself, "I wish I was skilled enough to build that".
I may be close to starting mine, I just have to find someone to fund it.
My ultimate project is a first class heirloom pool table. I don't have room for one at my house, I'm not sure I would even play that much if I had one. I just want to build one. But after looking at a few web sites on the subject I have come to the conclusion that I am going to need to find someone who wants one, so they can fund it and I can build it for them. There's a guy at work who is very interested, but is still a bit apprehensive.
So, what's yours? If you have already built it, was it a satisfying experience? Would you do it again?