I'll attempt to answer, might change my answers later!
1. one practical bike?
Leaning towards my Genesis Day One, a Joy to ride though a bit silly having one gear! Compromise to off road but it will just about do it.
2. what one bike would you buy?
I think this would be a steel CX bike prob with gears something like a Genesis Croissant de Fer, pimped naturally! OR a good light 29er
3. Finally, regardless of practicality, which one would you not be able to let go of?
I used to think I'd struggle to sell any of my retro's but I'm not sure now I'd worry more about replacing them if it was a horrid mistake to sell, nice to get down memory lane trip now n then on lightweight and fun old sketchy MTB's. Saying that I aim to keep as many of them as long as possible, sooo... I really dunno. I'd eventually miss a good capable 26er such as my Cotic if 29ers don't tick all the boxes which I expect they wouldn't.
Hope the GF doesnt see this thread!
Public declaration I could trim to 1 or 2 bikes! Viva la variety!