In reply to aln:
It is interesting how many foodstuffs are reputed to be the best in their class and also the rarest or most expensive.
With coffee, I forget the name of the rarest type of bean but you hear that coffee experts rave about it having the best flavour.
Same with chocolate, there is one variety of tree which is rarest and surprise surprise, we are told by experts that it produces the best flavour of chocolate.
Truffles are also the same. They are the rarest and most expensive fungus which yet again, we are told have a wonderful flavour. (Actually, they are revolting, in my opinion.)
Even fillet steak or rack of lamb. It is the smallest appreciable bit of an animal and once again, we are told it is the best.
And so we come to grouse. I think it is genuinely rare as it only thrives in the semi-wild moorland environment and is hard to harvest (ie shoot) whereas the other things you list are either vermin (rabbits, hare or pigeon) or are basically farmed nowadays (pheasant and partridge).
I do think there is a lot of unwitting snobbery, hype and one-upmanship going on here. Can it really be that in all these foodstuffs, the rare and expensive version has the best flavour? I doubt it.