Phenomenal Classical Recordings

When you're ready for Mahler get the SACD releases by the San Francisco Symphony Orchestra under Michael Tilson Thomas. Start with Symphony #1, because everyone loves that one.
I can verify that the Naxos CDs of Haydn's quartets are very good indeed. You can buy the whole 23 CD set for about $130, but if you just want to dip your toe in, I'd recommend the Op.9 (2,5,6) and Op.76 (1-3) which are great examples of his early and late style.

Cheers.

I'm an unabashed Romantic. I love the Romantic-and-later-period big-orchestra pieces. My favorite composer is Gustav Mahler but I love much of the music from mid-Beethoven* on. Perhaps my most-listened-too recording, however, is one of Gustav Holst's 'The Planets'. My absolute-favorite recording of it is by Adrian Boult and the London Philharmonc Orchestra (which many of us call the LPO), on EMI, CD #72435677492.

Enjoy your listening. I expect in a year or earlier you'll be able to say that you most love the music of a certain period...Baroque, Classical, Romantic, modern, etc. Then you'll start to hear and prefer certain performances of some pieces.

*--Ludwig didn't start writing anything for me until the 4th piano concerto and the 3rd symphony. His earlier stuff sounds too much like Mowart and Hayden...a little too neat and organized and not dramatic enough.
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If you're a Beethoven nut, as I am, you have to try 2 CDs: Both the Penguin Guide and Gramophone recommend them in their highest category for both recording and performance. They are just stunners:
1. Beethoven's 5th and 7th Symphonies - Carlos Kleiber - DGG and also available on SACD
2. Beethoven 6th Symphony - SACD - Bruno Walter