Belgium
Some have mentioned Belgium as a place to avoid. I 've always found Belgium a pleasure to fly through (I only everlanded once). The airspace makes sense and ATC are very helpful.
The Netherlands however is a right pain to get through, with their ridiculousairspace construction, which even the most helpful ATC can't do much about. If the weather is murky, which itoften is in those areas, I now route around through Belgium and Germany to stay at a decent height.