Rather that rattle off a list of my favorite pieces of classical music (we’d be here all day), I would advise you to do a couple real-world things to inform yourself so you can figure out what you like (of music on-hand now) and what might thrill you in future purchases:
1 - Take the time to read the liner notes. With classical it matters greatly who the composer is (of course) and who the performers are (of course, again) -- but in the larger sense, it matters what era of classical music you’re hearing (ie, baroque style vs classical style vs romantic style vs things that came in w/the 20th century).
2 - And when you hear something that thrills you, note what instrument it is (or instrument grouping: chamber vs symphonic). If you find yourself loving the cello (for example), there were fabulous pieces written for it all over the past 300 years
3 - And finally, note any composers whose works appeal to you, and any performers who impress you most. You can later on pick up more recordings based on those factors.
A friend was here yesterday and really liked a classical recording I was playing (an orchestral piece by Frederick Delius). I don’t know him to own any classical, so I asked, "If you like this so much, why don’t you own any of it?" His answer baffles me: "Because I don’t know enough about it to know what I would like."
All he has to do is spend some quality time youtubing classical of various types and eras--and pretty soon he’d have a very good idea what he likes. Your 300 recordings are the equivalent of that.