Most common epicenter of a radicular cyst is:
In most cases, the epicenter of a radicular cyst is located approximately at the apex of a nonvital tooth. It occasionally appears on the mesial or distal surface of a tooth root at the opening of an accessory canal or infrequently in a deep periodontal pocket. 
Oral radiology White and Pharaoh; 7th ed. Page no 335
Distal surface of tooth
Apex of tooth involved
Mesial surface of tooth
Deep periodontal pocket