At the time it happened, John Lennon.
Then decades later, it seemed George Harrison's death had a greater impact.
It can depend on how much time goes by after one celebrity's death already shocked you. Also after you get used to the fact decades later, you can forget how much it did shock you.
Right now a lot of the celebrities that Baby Boomers grew up with in the 1960s and 1970s are in their later years. Glen Campbell recently died. So did David Bowie, and Prince.
It seems that Sinead O'Connor may be next soon even though it is because she is reportedly suffering from suicidal tendencies .
Loretta Lynn has been hanging on for a long time even though she has dealt with illness wrought by old age.
Helen Reddy reportedly was placed in a nursing home because of dementia.
Kenny Rogers reportedly isn't feeling well due to older age. Also Julie Andrews can't sing anymore, Linda Ronstadt, Bridget Bardot, and some others who we may soon find ourselves being shocked about