Yay Belgium!
For a second leader, Leopold II is an interesting choice. He colonialised Congo (as his personal property, only after his death it became property of the Belgian state). He had the biggest personal fortune of all Belgian kings, mainly because he exploited Congo for it's riches, using the local population as slaves. He had a healthy interest in women, if I'm not mistaken the larger part of his inheritance went to one of his girlfriends. He was as dictatorial as one could be in a democratic state like Belgium, which wasn't much, but he made up for that lack of power in Belgium by oppressing the locals in his personal playground, Congo.
And he had an enormous beard.
In short, an interesting character, and probably easier to put in Civ than any of the other ones, though if you can do Boudewijn, you can do any of them I guess, that guy never did anything
Other kings of Belgium:
Leopold I: didn't do a thing.
(Leopold II goes here)
Albert I: most noteworthy feats: not surrendering in WWI, falling to his death
Leopold III: married two times (teh horror), collaborated during WWII, forced to abdicate.
Boudewijn: you know him

Albert II: laughs a lot, his most noteworthy quality is that he knows the power he holds as king (which is to say, none), unlike his sons.
Next in line: Filip. Will probably be known as the last king of Belgium, unless he learns very soon how nót to **** things up as a member of the royal family.
There ye go. But i'd still go with Leopold II, he's interesting. Well, as far as Belgian kings go...