What's the safest over-the-counter pain reliever?

Any. All otc pain relievers are safe when taken by a health person at the prescribed doses.
Depends. It depends on any other problems you may have, but in the absence of liver disease tylenol (acetaminophen) in recommended amounts is safe. Also, in the absence of kidney or stomach trouble, Ibuprofen is safe as well. Neither should be used in large amounts or for a prolonged period of time without consulting a physician.