looking for ideas on must have classical music

This link may be of interest to you.
https://www.nytimes.com/2019/04/19/arts/music/classical-music-piano.html?smid=nytcore-ios-share

@ott, "Classical music consists in a vast array of sub-genres, and almost limitless modes of expression."

Yes that's easily forgotten when you want a similar Classical thrill to some hard rock or harmonic pop.

Speed/ tempo is probably the main criteria I use to categorise music. I don't really enjoy too much of a mix match when it comes to tempo because that doesn't help to create a particular mood. At least not for me.

Based upon the nature of your question it might help to think in terms of time periods then concentrate on great examples within each period. Specific recording are unimportant as you will over time develop your personal favorites. Examples might include....
  
1. Medieval (Gregorian Chant)
2. Renaissance: William Byrd, Thomas Tallis, and Giovanni Gabrieli. 
3. Baroque: J.S. Bach and Antonio Vivaldi.
4. Classical: Mozart, Haydn, early Beethoven
5. Romantic: Beethoven, Brahms, Tchaikovsky, and many others
6. Impressionists, Debussy, Ravel, 
7. 20th Century: Bartok,Stravinsky, Gershwin, Bernstein, Barber to name a few.

As you listen you will gain the ability to hear something unfamiliar yet still have and idea of what time period it may have been composed.  This ability will find you well on your way to a lifetime of discovery and enjoyment.  HAVE FUN!      

Some people come to classical with a bunch of composers , others
by following in love with only one or two .
For the latter,
under 50, Bach and Schubert,over 50 , Bach and Brahms .Brahms often has a powerful effect on otherwise educated older folks .

Concerto for Violin and Orchestra--Philip Glass