So many belfries! My wife and I visited Belgium in August 2017, and decided to see a representative sample of the belfries on the list. In the end we saw two cathedral belfries (in Antwerp and Leuwen), a town hall belfry (Antwerp), two marketplace belfries (Bruges and Ypres), and two standalone belfries (Tournai and Mons).
Of these, I think Antwerp cathedral, Bruges and Tournai are the most impressive mainly due to their height and imposition on the surrounding area. And if you're going to climb one, don't make it the one in Bruges! It's 10 euros, while climbing in Tournai is 2 euros.