Is it legal for me to stand on public property near a doctor's office and express my negative feelings?

I don't want to be sued for libel/slander, but feel a dentist who treated me is a non-caring, all about money jerk.

Does anyone know (the dentist is in Rockledge, PA. Would I be within legal rights to hold a sign saying "I would NEVER return to this dentist."

Thanks.
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