looking for ideas on must have classical music

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

Incredible feedback and i am extremely grateful for all of it.

all have my most sincere thainks.

One of my favorites is Tchaikovsky‘s violin Concerto in D.  I have a few versions.

Another amazing collection of work to consider is the entire collection of Beethoven’s piano sonatas played by Mari Kodama,  hi-Rez and high fidelity recording available by pentatone classics.

Vivaldi’s four seasons is extraordinary popular and for good reason, it’s quite good!

The Bach cello suites by Yo-Yo Ma. He has recorded it three times in his career, the most recent one just came out last year and it is excellent.

 Check out anything by Julia Fisher on pentatone.

Also check out some of the Wilson audiophile  recordings available at acoustic sounds.

good luck, have fun!