A 45-year-old man presents with difficulty hearing in his right ear and tinnitus. Otoscopy reveals a dull tympanic membrane with visible fluid level (air bubbles may be seen) behind the eardrum. There is no pain or discharge. Rinne's test shows bone conduction better than air conduction (BC>AC) in the right ear, and Weber's test lateralises to the right ear. What is the most likely diagnosis?