A 27 year old man presents with a 1 week history of a right-sided neck lump. On examination, he is apyrexial with stable observations and a normal throat examination and no cervical lymphadenopathy. You palpate a smooth, fluctuant lump in front of the anterior border of the sternocleidomastoid muscle. It does not transilluminate and there is no movement upon swallowing.
What is the most likely diagnosis?