For those of you who are learning French/Spanish: what are some sounds that you find difficult to pronounce? I.e the French/Spanish "R", the "U" in French, etc.

I'm asking because I'd like to create some more videos on my YouTube channel about how to pronounce difficult sounds in those languages. I'd like to see what sounds people find difficult so that I can figure out what to make my upcoming videos about :)

I recently made a video about the "U" sound in French, and I am undecided about what letter(s)/sound(s) to choose next. All suggestions are welcome!