I realize this question sounds a bit subjective but in the English-speaking world I feel pretty confident that most people would agree on Glasgow and Northern Ireland area accents being the most in need of subtitles in other parts of the Anglosphere.
My German is quite rudimentary but I've been able to follow the gist of the conversations pretty much everywhere I've visited in Germany, Austria, and Switzerland. Yet during a long evening in Oberwallis I couldn't even guess what a single word was. I felt all the endings were foreign and couldn't relate its sounds to the sounds of "normal" German at all.
So is Walliserdeutsch for German speakers the equivalent of Glasgow/Belfast for English speakers?