Jean-Claude van Damme... erm... Tintin?
Surprisingly, the Belgian Embassy in London has an entire page devoted to the question of the “five famous Belgians” game. (It’s good to see that they have a sense of humour.) If you’re a pedantic Belgian, though, you’ll probably want to resort to Wikipedia’s “List of Belgians” page, which tops out somewhere just short of a thousand.
But here’s the most easily defensible answer:
Jean-Claude van Damme; Kim Clijsters; Herge (creator of Tintin); five-time Tour de France winner Eddy Merckx; and Adolphe Sax, inventor of the saxophone.