I just said in conversation:
Le plus fort dans tout ça, c’est qu’elle ne s’en est jamais rendu compte, alors qu’on croyait tout laisser transparaître d'une façon on ne peut plus évidente !
In English, you use the cliffhanger-ish phrasing "The beauty of it all is that ..." to introduce a punchline. The word "beauty" here has no relevance to « physionomie », but rather denotes one particularly good quality or interesting element that stands out from the rest.
I cannot seem to find any dictionary entry or webpage that mentions "Le plus fort dans tout ça, c’est que ..." as the equivalent of "The beauty of it all is that ...". Without much to back up my assumptions, I'm not sure of this.
How do French speakers usually express this idea?