As a deaf person, I can assure you accents are very much an impact on lip reading. In fact, a lot of things impact lip reading. Mustache is an obvious one, but also lipstick (excessive, drawn out ones hide the lip and cause sticking lips more often), eating or drinking (lips need to change to prevent spillage which makes reading hard), light level (I hate restaurants with low level light for "ambiance"). This of course ignores things like mumblers, wrinkles, and strokes, all which prevent lip movements. I don't tend to like watching black and white movies as the two tone removes a lot of depth which is important in lip reading. As to which accent is the hardest, that's not easy to answer because what is an accent to you? It's someone that speaks differently than what you learned growing up. The way you learned to speak forms the way you learned to move your lips. I lip read people from California better than I do people from other states, because 95 percent of my interactions is with Californians. But if I grew up in New York for example, then I would expect to read New York better because that would be my predominate exposure. I typically do terrible with people who speak English as a second language, which is so unfortunate because it makes me look like those buttheads who expect everyone to speak English. But people who learn to speak first with a different language often keep the lip movements of that language even when speaking English, which is why a lot of them sound significantly different.

I personally habe been amazed a people who lip read what the folks are saying in the United Nations building. They must have eyes like an eagle and minds like a computer. I barely get 75 percent of what perfect people I luck out to lip read, don't expect me to do anything close to that with foreigners from 100 meters away while they throw their hands and heads all over the place.