According to the web site "Famous Belgians" http://www.famousbelgians.net/ this is their list of the ten most famous Belgians:
1. Eddy Merckx; 2. Adolphe Sax; 3. Herge (George Remi); 4. Audrey Hepburn; 5. Plastic Bertrand (really?); 6. Peter Paul Rubens; 7. Rene Magritte; 8. Georges Lemaitre; 9. Albert Claude; 10. Leo Baekeland. Stretching a point there with Audrey Hepburn and Belgium didn't exist when Rubens was born.
I would like to add another name to the list; Dani Klein. She is not famous but she ought to be. She was the singer of a very popular music group called Vaya Con Dios, one of the most successful Belgian music acts ever, having sold more than 10 million albums and more than 3 million singles. They were active from 1986 until 2014 when they disbanded, Klein thereafter continuing with a solo career.