Belgian, Molenbeek is as similar as Saint Denis in many points (not only the bad ones fortunately).
Saint Denis is as Molenbeek not only a bad place where Jihadists hide, but unfortunately this is also the truth: both locations have nowadays their reputation heavily tarnished, and it is partly true that bad things happen there.
However Molenbeek must be also, as you say, a very nice place, no doubt on that, so is Saint Denis, with an incredible story, the very place where almost all the kings of France and of the Franks are buried there from Clovis to Louis XVI inside the master piece of the Gothic Art that is the Basilica of St Denis and its royal necropolis, if you visit Paris one day, I advise you to go to St Denis, it is sadly less popular than Notre Dame, but it is actually better to go to St Denis than Notre Dame, since this basilica used to be more important compared to Notre Dame.
https://en.wikipedia.org/wiki/Basilica_of_St_Denis