Impossible task, given numbers of recordings of so much great music... but would be helpful to know for some things whether "classic recordings" regardless of sound quality, or more sound quality, is emphasized (like, Furtwangler's Beethoven could be a separate thread altogether based on "best performance" and "best-recorded performance" and reasons why Furtwangler was/wasn't the greatest Beethoven conductor of all time etc etc).
Or, say, comparing Milstein's Bach with Hilary Hahn's leads to all sorts of discussions about what's desired.
Also, interest in "historically informed performances" or not? (Say, with a Beethoven cycle again, for example. Early in this thread it seems Karajan was just a given. SO many other options out there... )
Sorry if this is just muddying waters, but the lists can reach thousands of recommendations without anyone blinking!
And if you want something super-simple, then I'll just pick one: Milstein's DG recordings of Bach's Sonatas and Partitas for violin solo.