Most frequent site of branchial cyst is at
BRANCHIAL CYST

It arises from the remnants of second branchial cleft. Normally, 2nd, 3rd, 4th clefts disappear to form a smooth neck. Persistent 2nd cleft is called as cervical sinus (of His) which eventually gets sequestered to form branchial cyst. 
Swelling in the neck beneath the anterior border of upper third of the sternomastoid muscle. It is smooth, soft, fluctuant, often transilluminant with a sensation of ‘Half-filled double hot water bottle’.
It is equal in both sexes. Even though congenital, it is seen in late adolescents and early 3rd decade.
It contains cholesterol crystals which is from the lining of mucous membrane which contains sebaceous gland. Cheesy toothpaste like material is typical.
Upper third of posterior border of sternocleidomastoid
Lower third of anterior border of sternocleidomastoid
Upper third of anteromedial border of sternocleidomastoid
Supraclavicular fossa