You know what? I'm going to leave out any particular works deliberately. All of them are very important and worht knowing about. Look them up yourself. One is famous because she married John Lennon. Another was her ex-husband when she married him. Another is Beck's grandfather. All created visual monuments.

Jasper John
Al Held
Nam June Paik
John Cage
Yoko Ono
Claes Oldenberg
Tony Smith
Dennis Oppenheim
Al Hansen
Eva Hesse
Judy Chicago
Jenny Holzer
Robert Smithson

These are great artists who have rightly enjoyed high reputations. Their products are diverse and, because of the times we live in, sometimes ephemerial. If you even know who they are you can usually appreciate the diversity of Modern Art, but what Miss Ono did with John Lennon should be seen in a context you probably aren't familiar with.

It's worth taking the time to look up each of them individually.