It's hard to know what boomboxes are genuinely rare, because company data on these things is long gone, if it ever existed. But give yourself a few years of searching - in the wild, on eBay, and on boards like this - and you'll eventually get a pretty good idea of which boomboxes turn up often and which appear only once in a blue moon.
Of course, wanting a box just for its rarity seems a bit foolish, Unfortunately, I'm guilty of that, because I like to be different. (Sorry.) There are other folks who like to find (and flip) rare boxes for profit. But the best (and most satisfying) route seems to be simply to buy what looks and sounds good to you, no matter how scacre or plentiful it is, and no matter how many (or few) other folks like or want it. Be an individual, and try not to crave something just because other collectors crave it. Since beauty is in the eye of the beholder (same goes for sound in the ear), as you collect, you're likely to snag a rare box or two along the way without even trying... just because you'll be one of only a few people who like it.