Reboot your Classical Collection

I know many of us are collectors as well as fans, it's just the nature of the genre. This thread is for anyone interested in what other collectors feel would be a good starting point to a collection for all genres and eras of classical music. Opera, chamber, orchestral, songs - choose whatever it is you like most and whomever you want to recommend. The idea, though, is to recommend sets or cycles of works - I became interested in creating this thread after reading the various recommendations for [...]