"I’ve never been very comfortable with public speaking. Realizing this was a problem, I asked my previous employer if I could work the front desk to increase the interaction I would have with patients. Since then, my public speaking has improved, I've learned my job even better and my customer service skills are always improving."
This question is a great opportunity to put a positive spin on something negative. Don't answer the interviewer with something cliché – joking or not. Instead, try to use a real example of a weakness you have learned to overcome. Choose a skill as your weakness as you can learn skills. Avoid describing a characteristic as your weakness as it is harder to change someone.
"My last employer taught me the importance of effective time management skills. She was extremely deadline-driven. Her no-nonsense attitude pushed me to work harder, and to meet deadlines I never even thought were possible. I definitely learned a lot from her."
Don't fall into this trap of negativity. You may or may not have enjoyed working for your last employer, but now is not the time to throw them under the bus. Avoid being negative. The best way to answer this question is with something that you learned from your employer.