Most common site for the initiation of otosclerosis is -
Most common type of otosclerosis – Stapedial otosclerosis
Most common site of otosclerosis – Fissula ante fenestram (i.e. just in front of oval window)
Most common site for stapedial otosclerosis – Fissula ante fenestram (i.e. just in front of oval window)
Most common site for cochlear otosclerosis – Round window

Ref. Dhingra 5/e, p 97-98, 6/e, p 86-87
Foot plate of stapes
Margins of stapes
Fissula ante fenestram
Fissula post fenestram