In terms of national carriers, weren't MEA using only 707s / 720s for a while? Will check the books tomorrow.
Also Austrian Airlines; after retiring the Caravelles they flew only DC-9s for quite a while until the A310 arrived.
Lots of secondary / charter carriers only operated one type at a time but one that comes to mind is Transavia who have only ever operated variants of the 737. In that regard they even beat Southwest who dabbled with 727s.
Last edited by El Bunto; 6th May 2018 at 21:47.