BASILICA OF THE HOLY BLOOD, BRUGES
If you're only going to visit one church in Bruges make it this one. The Basilica of the Holy Blood is not only impressive for its blending of Romanesque and late Gothic architecture but also for the sacred relic kept inside. The upper chapel is home to the famous vial for which the church takes its name; said to contain a drop of Jesus Christ's blood that was brought back to Belgium after the Second Crusade.
The interior of the church is a dazzling riot of gild work that was completed in the 16th century and provides ample reason for a visit even if you're not interested in holy relics.