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!!
blindjim
A couple of suggestions for the OP and all the contributors to this thread: If you stream, check out the IDAGIO streaming service for Classical music exclusively.
Many search opportunities and wonderful suggestions for new recordings and even particular moods.
Also,  I invite all of you to check out the thread “Classical Music for Aficionados,”
Talk about Classical music.
Where have you all been?

The suggestions already offered are great. I would add: had a really wonderful experience learning about classical music, as well as it's historical precursors and variations, from a course offered by The Teaching Company, "How to Listen to And Understand Great Music," by Robert Greenberg. I had been a lifelong rock and jazz listener, and this series opened by eyes; it was a revelation. It's hard to appreciate the richness of classical music without an understanding of its form. The course is a wonderful and detailed introduction, also with lots of samples of varied genres. 
It's costly at its full price, but if you get on their email list they discount considerably and frequently. It's worth the wait.
Enjoy!
@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!