looking for ideas on must have classical music

Hello classical devotees!

A friend had me rip quite a few of his CDs for his trip abroad. most were classical, and many were like 'best of' compilations. one or two were whole symphonies.

armed with these at least and no others I felt it time to wade into the classical waters and increase that genre in my library.

what then, are your fav, 3 or 4, go to, gotta have composers, movements, and or conductors out there on CD at least?

if also available in HD or otherwise, please point towards them as well, if you don't mind.

your input is sincerely appreciated and this input will initiate my list for current and future additions to the catalog, so again, thanks very much!!