my vote goes to Lufthansa Miles and More.
it has some superior aspects compared to SAS EuroBonus and Finnair Plus for example. Especially you get free flights much quicker, 20,000 miles will take you basically anywhere in Europe; Armenia and the lot. Ok, these flights are not bookable until 2 weeks before departure. With SAS you need a minimum 30,000 miles to fly somewhere in Europe, Scandinavia excepted (12,000 miles). With M&M you also get miles on all partners´ flights. With Finnair Plus this is not the case - certain fare classes of BA
for example did not give any of their "plus points", at least until recently. Nowadays they basically give only half of the points they used to anyways - this is applied to all discount fares. That's one of the reasons I practically never pay for flying with them.