This is a tough question as I'm still learning (well re-learning) about the bikes I could only dream/read about in the magazines when I was a lad. My first mtb was a Raleigh Cajun and I've never seen another. If one came up I'd consider it for nostalgias sake alone.
I had an rts 2 bitd and its what lead me to this site, after attempting a restore I found it had cracked so was looking for another. Ended up finding a team rts and went through various builds but it is now quite modern wearing m970 xtr, crossmax ceramics, fox forks and carbon bits and pieces. This is my unicorn but as its not retro anymore I've got another waiting in the wings to drape in m900, which is the bike I could not afford bitd (I couldn't have afforded the rts 2 either but after having a couple of bikes stolen that cost me a few hundred quid, a new for old insurance policy covered it somehow).
Other unicorns are:
Pace due to mint sauce (one day I will actually finish that build)
Dekerf - never heard of them before finding this site, but own one now and will one day get around to that too
Amp - was always amazed by them bitd and have had a few that I've never finished building, so the litespeed will hopefully scratch that itch
That's about it - I have other bikes I've bought due to their cache (Klein, to eddy for example), but the Klein is too small and too rigid for an old bloke like me, and the fat, well its still in its box 3 years later and whilst I was excited to own it, I can't inspire myself to build it as I really want an aqua fade yo eddy in m/l, and it isn't one sadly.
I could go on, as the more time I spend on here, the more bikes I see and like and want, but I should focus on the few and part with the many so that I can ride, feel the breeze through my hair rather than spending most of my time shifting boxes of parts, frames and wheels, which get in the way of what it is really all about.