Isn't the ultimate bike unobtainable and what you haven't got (yet)?
This is obviously a rhetorical question but for me every time I think "right, that's it" another idea forms in my head - I mean of those mentioned above I've got the Vit T and Yo (2 for goodness sake) but I currently want a steel Stumpjumper (as per my signature - anyone?) and the fact that it appears elusive only wants to make me want it more. Sate that hunger and on I move!
All that said, the one I really wanted BITD and haven't got or had is an RC100