Did J.K. Rowling ever reveal what scares her most? In other words, what form her boggart would take?
As a matter of fact, she did (same interview she indicated what her Animagus form would be).
Q: What shape would a Boggart take if it wanted to scare you? How would you defeat it?
A: J.K. Rowling responds: I think I'd probably have Aragog, as Ron did. I hate spiders.
Src: Transcript of J.K. Rowling’s live interview on Scholastic.com October 16, 2000
Her Boggart changes over time, usually reflecting what she is currently writing.
1970's - Scary librarians
@nancy_whiskey: I'm not sure the inventor of Madame Pince could be taken down by a librarian's disapproving look!
@jk_rowling: You're so wrong. She sprang directly from my childhood fear of scary librarians. The kind who hate kids.
1990's - Harry Potter being unpublished
@givemhelldarlin What kept you motivated? Did you ever just feel like giving up at some point?
@jk_rowling: I wasn't going to give up until every single publisher turned me down, but I often feared that would happen.
2000 - Giant spiders
Question: What shape would a Boggart take if it wanted to scare you? How would you defeat it?
J.K. Rowling responds: I think I'd probably have Aragog, as Ron did. I hate spiders.
"About the Books: transcript of J.K. Rowling's live interview on Scholastic.com," Scholastic.com, 16 October 2000
2001 - Dementors
Child: If you saw a Boggart, what would it change into and how would you destroy it?
JKR: Erm ... either a spider, like Ron sees when his Boggart turns into a giant spider, possibly a Dementor, because I think they're the scariest things I've written.
Blue Peter (cBBC), 12 March, 2001
2005 - Dead relatives / Being buried alive
What would a Boggart turn into if you stood in front of one?
The same thing Molly Weasley saw when she let a Boggart out of the locked cabinet in 'Order of the Phoenix' – my loved ones, dead. If, however, you are talking about a fear that concerns only me, it would be being buried alive. I'm not sure how a Boggart would represent that; perhaps turn into a coffin around me? Urgh.
J.K. Rowling's old website - FAQ
2014 - Forgetting Harry Potter details
@jk_rowling: I had a waking nightmare where I met a Harry Potter fan who quizzed me on a sub-plot & I couldn't remember what I'd written
2016 - Public speaking
@SeaBowmaster: what terrifies you JK?
@jk_rowling: Public speaking. I agreed to read live at the opening ceremony of the 2012 Olympics in the belief that it would either kill or cure me.