Too many people think of him as ordinary and earthy, compared to Mozart or Beethoven. Yet he composed amazing amounts of pathbreaking, first-rate music, and it wears remarkably well upon repeated listenings.
My approach to Haydn is pretty simple:
1. Some of the early piano music is boring, but a simple availability metric will point you to the best material. The deepest are the six last sonatas, and most well-known performances are quite good. Ax, McCabe, Kalish, Richter, and Brendel are among the first choices, Jando (Naxos) and Buchbinder are good enough to listen to but not preferred. By the way, piano > pianoforte, there was no great stagnation.
2. Listen to as many of the string quartets as you can, with preference given to Opus 76. On average, the later opus numbers are better, yet Op.9 and Op.20 still are worthwhile.
3. Listen to the London Symphonies. Again and again. All of them, Dorati being one option for conductor.
That’s hardly the only wonderful Haydn, but those are the pieces that work best through recordings. See the choral and vocal music live. Most of the concerti bore me, as do the piano trios. Many of the earlier symphonies are good, including the Paris set and the “Sturm und Drang” period, but unless you have lots and lots of time I say focus on the London ones for now.
As the years or decades pass, you will realize you have been underrating Haydn.