My vote goes out to the SAS "Fly 5 get 5" award when they were promoting the Star Alliance in 1997.

Each member had the same qualifier with mild variations: Fly at least one paid flight on each of the original 5 *A members (UA, LH, AC, TG, SK)

For your trouble, SAS gave you a free RT for 2 people on EACH of the 5, systemwide, with one stopover permitted in each direction.

PLUS - you didn't have to credit the qualifying flights to your SK account.
I gave up my UA number, and qualified for their bonus as well (50K miles)

I took in Vancouver, Toronto, London, Copenhagen, Oslo, Tel Aviv, Beijing, Bangkok, Perth and Buenos Aires...