The greats, and I may be accused of being too absolute and deriding not so good music, are generally considered great because they are great. It seems to be a pastime of armchair critics to try to delight in knocking the established pantheon. That having been said, some composers, for whatever reason, whether that be the saturation point of listening to certain works or composers or whatever, are unaccountably neglected, or if not, even somewhat undervalued. A classic case IMO is Faure, hardly an unknown (except if you ask the average person on the street, then he likely will be literally unknown). It is a question of relatives, not absolutes (and you should meet my relatives). It is also a fascinating parlour game, a pursuit of the leisured and sated (and also the ambitious musicologist- Q What is a musicologist? A A person who can read music but can't hear it).