Yet another reason why Belgium is so worth visiting - the Ghent Altarpiece. This work is said to be on many art historian's list of their top ten paintings in art history.
Though Jan van Eyck did not invent oil painting, "he was the first master art to exploit its true capabilities." (Noah Charney, "Stealing the Mystic Lamb: The True Story of the World's Most Coveted Masterpiece." Charney calls it "the most important painting in history."
It was mostly completed 1426-1432. The painting has gone by various names usually have the word "Lamb" in them, from the central panel, but today it's often just called The Ghent Altarpiece (everyone will know which one you mean). It's a massive 20-panel altarpiece in St. Bavo's Cathedral in Ghent (St. Baaf's in the local language, as Tom pointed out below).
Rick's guidebook describes it as "the most influential painting in art history", as it was arguably the first work by a master done in what might be called the perfected technique of oil painting. This work alone makes Ghent well worth the easy day trip from Brugge, about a 30-minute train ride away. The minute detail in such an enormous painting is just one of the outstanding features.