Here's a can of worms. We could all pick one record but does that really work, eg some may pick instrumentals, which I tend to dislike, but is that a good introduction. If I had to pick one i'd g with 'Tainted Love' as most people will know a different version, if they prefer that they will never get it, however anyone with a bit of soul will say ok, then you can start to increase their knowledge as the potential playlist yo would create for them will have all sorts of things on it.
The other worm in the room is the term 'Rare Soul' ie i'm sure there are some reading who consider themselves rare soul fans/collectors but would be turned off by some very rare Deep Soul or Modern Soul. At the end of the day it has always just been Soul to me, some I love, some I like, that doesn't mean we have to agree as there is plenty out there. However, to me anyone who buys something just because it is rare is an investor, but hey, 'to each his own'.