There was a post on this subject just a few weeks ago, but it was full of exact passenger numbers and info, so look it up. I think it said that the busiest city pair is NY-LA, busiest international route is London-Paris, and busiest non US domestic route is Paris-Nice or Sydney-Melbourne. There was a US top 10 as well, which was fascinating. Check the archives.
fly Saha Air 707s daily from Tehran's downtown Mehrabad to Mashhad, Kish Island and Ahwaz