We explored some very good ones in Belgium! A couple of stand-outs:
Brugge's Groeninge Museum: it's not very large but has a valuable collection of Flemish works spanning 6 centuries. Among the Flemish Primitives are Van Eyck's ridiculously detailed "Virgin and Child with Canon Joris van der Paele” and the "St Nicholas Altarpiece" by a Primitive painter known only as the Master of the Legend of St Lucy. There are also some wonderful paintings of a long-ago Brugge and her citizens, nicely outfitted in the fashions of the eras.
Brugge's St-Janshospitaal: an art museum isn't the primary function of this one - which has a fascinating history! - but its excellent collection of six Hans Memling works is the delicious icing on this cake. The “The St. John Altarpiece” and “Shrine of St Ursula” are not to be missed.
Antwerp's Museum Mayer van den Bergh: I enjoyed much of this 19th century collector's treasure trove of Gothic and Renaissance works from Northern Europe very much! The building, an early 20th-century facsimile of a 16th-century townhome, was built specifically to showcase the collection of paintings, carvings, sculpture, illuminations and whatnot. Lots of folks come here for Bruegel the Elder's "Mad Meg" but there's something there for almost any art lover. I especially liked some of the carved pieces, such as this one:
https://www.wga.hu/support/viewer_m/z.html