Favourite PvdA PM of The Netherlands?

[DavidB.]

Drees, of course.

No disagreement here. The country needs more PvdA politicians like him.

As for the differences: Drees was a typical post-WWII style social democrat and arguably one of the most popular Prime Ministers ever. Den Uyl was somewhat of an "old leftist" too, but he had to deal with a party being taken over by the New Left movement. He led the most left-wing government in Dutch history -- he also led the PvdA so far to the left that it became huge, but uncoalitionable from 1977 until 1989 except for a brief period in 1981-1982. Wim Kok was your average Third Wayer who said the PvdA "had lost its ideological feathers" and thought that was a good thing. His purple governments, which introduced some of the most progressive legislation in the world (SSM, euthanasia), were pretty popular but cannot be judged without taking into account the Fortuyn revolution in 2002, just like Obama's presidency cannot be judged anymore without taking into account the Trump win.

[DavidB.]

In what way was Den Uyl's "the most left-wing government in Dutch history"? Is it just in terms of New Left stuff, or was there also some serious social-democratic reform? I voted Drees because I thought he's the one who did the most to build the Dutch welfare state.

True. Drees built the Dutch welfare state. Den Uyl's government is usually seen as the most left-wing in Dutch history because of the dominance of the PvdA and the presence of both PPR and D66: symbolism matters. The heavy New Left influence in the PvdA of that time also plays a big role here. In terms of actual socio-economic policy, however, I would say no real social democratic reforms took place. The government collapsed because KVP ministers could not agree to the government appropriating private lands for new housing.