Been the only passenger in first class/business class cabin at least three times that I can remember. All three of these flights had one premium cabin (marketed as either first class or business class) and economy.

Royal Jordanian from AMM - TLV on a regional jet.

Druk Air from KTM - PBH on an A319, and

Malev from BUD - KBP on an 737.

Also had a flight on UA from JFK to GIG on a 747 where there were only 6 passengers on the entire plane and a CX flight from YYZ - HKG on an A340 with less than 10 passengers on the plane during SARS. Was in economy on both these flights, obviously all passengers had plenty of seats to choose from.