Salieri: Requiem in C minor - Sanctus & Benedictus
While we're on the topic of neglected composers, how about poor Antonio Salieri? Had he not been cruelly libeled by Peter Shaffer in the play & film Amadeus, it is quite possible that his music would not be performed at all (incidentally, Shaffer did no favors to the memory of Mozart either, who was the purported protagonist of his story). The truth is that although Mozart had some suspicions about Salieri when he first arrived in Vienna, the two eventually developed a friendly and respectful professional relationship. Salieri, in fact, responded very favorably to Mozart's opera The Magic Flute, and in his final letter Mozart mentions taking Salieri to a musical performance in his own carriage. Needless to say, Salieri did not murder Mozart (nor anyone else that we know of).
The lovely piece below is the "Sanctus & Benedictus" from Salieri's Requiem Mass, one of his four Masses. It is, I think, a good example of why he was considered one of the finest composers of his day.