Most common cause of unilateral proptosis is-
*Thyroid orbitopathy is the most common cause of bilateral and unilateral proptosis. Ref: Head & Neck surgery 2nd/e p.1513
Thyrotoxicosis
Retinoblastoma
Intraocular haemorrhage
Raised intracranial tension
Diseases of orbit, Lids and lacrimal apparatus