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.