Dear Immanuel:
Thank you for this thread. I really agree with you: Haydn is very underrated. Sometimes I guess that people (pianists in particular) think something like: "Oh well... I have Mozart... I have Beethoven... Enough is enough..."

The music of Herr Franz have fabulous examples: the string quartets opus 20 n. 5, 33 nn. 2 & 5, 76 nn. 3, 77 nn. 1 & 2; symphonies (that's a very long list...) 6-8, 26, 44, 45, 49, 60, 82, 92, 96-104; some of his baryton trios; the concertos for cello; the great vocal works (The Creation, first of all; but also Missa in Angustiis, Cecilia Mass and other masses; Insanae et vanae curae). And, of course, his piano sonatas. My favorites are, according to Hob XVI: 20, 34, 36, 37, 48, 50, 51 & 52. And there are more piano music by him.
I don't say that someone might love Haydn more than Mozart or Beethoven, but the music of the older master is plenty of marvelous surprises and moments of absolute beauty and genious. Oh! Don't forget that both Wolfie and Ludwig were indebt troughout their lifes with him.

Best wishes! Let's play and listen to more Haydn!