Maybe some of the airlines in Myanmar. I have no personal knowledge of them, but if I had to make a wild guess as to your best chance for finding one where it was still legal, maybe there.

Your best bet might be to try an airline on which smoking is not allowed but where you might get away with it through either a bribe or staff indifference/incompetence. Aeroflot comes to mind. Or Saudi Arabian, Air India, Ariana, or Pakistan International Airlines. Maybe Tarom or Bulgaria Air. I have flown most of these, and feel like I could have snuck a cigarette if I really wanted to.