I've been fascinated with western European history for a number of years and visited many historical sites. Based on that I would say Belgium has a lot to offer. It has been the cross-roads and battle ground of Europe for millennia. If reading any period of European history Belgium has been involved some how, through Celts and Romans, Napoleon and the Kaiser; Belgium has been the playground for the big boys to throw their toys.
The UK has probably the most complex of histories, from an island off the coast of Europe suffering invaders and conquerors to the largest power the world has seen so far, only to then dismantle back to its own shores, it is quite a story.
I do like US history mainly due to the large quantity of archive material, most of its major historical events have been within the reach of film. To see a black and white picture of the US Civil war is so much more moving than seeing an oil painting or sketch of the Crimean for example.