Not Plastic Bertrand?
Tintin may be fictional, but Hergé wasn’t, of course.
Realistically, though, it’s the Muscles from Brussels.
I wish Simenon would be the answer, as then the most famous fictional and non-fictional Belgians could be associated with mystery novels!
Lots of good answers here.
Be honest: did anyone even know who Herman van Rompuy was without looking him up?
There may be some money riding on this for me…
I never heard of him.
Ambiorix. Obviously.
You realize that Hercule Poirot did not actually exist, right?
Yes, that is why I went for Albert. 
Jacky Ickx, outside the US, at least.
Hergé was the first Belgian that came to my mind.
King Leopold, unfortunately.
#4 on the list
Probably Charlemagne.
Without looking at the other answers: Eden Hazard.
I thought of Hergé and Leopold II before anyone (real) in the poll, and now I am reminded, I should have thought of Plastic Bertrand too.
Adolphe Quetelet is quite an important figure in the history of statistics.
Never heard of Herman van Rompuy before.
Julius Caesar
King Albert I
Kim Clijsters or Justine Henin.
Both women tennis players who were ranked #1 in the last 10 years.
Another vote for Hepburn and Rubens ahead of the people in the poll.
Rubens doesn’t count because Belgium did not exist back then.
Am glad Magritte is getting some votes! He’s who I think of first.
Yes, I knew, but that’s because I’m French, so we hear sometimes about Belgium.
Being French also means that the obvious answer to the poll is Jacques Brel, no contest (and Stromae should be on the list, now. He has a tremendous (deserved) success that goes well beyond the borders of Belgium)
Worldwide, I would have gone with Vandamme, but if Audrey Hepburn was Belgian, then I guess she wins.