I originally started this post with the words "Making a short list is easy", but as I started making that list, I realised that even culling down to three or five was hard...
I guess that if I was stuck with just one bike, it'd probably be a twin shock Husky WR400 - acceptable VMXer, and still a pretty good road-registerable enduro/trail-bike. Maybe an 84 KTM would do the job too. Or a complianced YZ250L like Brett Smith used to have.
But then again, there are a few bikes in my shed that I'd have a VERY difficult time parting with - the 94 TM125, the YZ125A, the YZ125X, the YZ250G, the DT250A that I've owned for more than half my life, etc... In reality, if it came down to it, my one bike would probably be one of those five.