It would have to be what a collector does not already have in a collection.
Lets take an M-90, C-100 or GF-7X7 as an example, if you have one you would probably only seek another as an upgrade (cosmetics or working order), as for non collectors I have to agree with Glue about the Disco lite, people just seem to love the looks of them. Desirability goes up and down, not to long ago everyone wanted Sansui's now the focus has shifted to the Bombeat 40.
In the end we collect for personal reasons and have personal "grails" we want, be it a childhood box or some other memory, I think these are the most sought after boxes to any collector.