Most common cause of unilateral parotid swelling in a 27yr old male is:
Pleomorphic adenoma or benign mixed tumor accounts for 80% of parotid tumors and 60% of all salivary gland tumors. Most common site is parotid gland though it can arise from submandibular gland, salivary gland of palate upper lip and buccal mucosa. Wahin's tumor occurs most common in males, but most common tumor in males still is pleomorphic adenoma.
Wahin's tumor
Pleomorphic adenoma
Adenocarcinoma
Hemangioma