What are some of the hardest words to pronounce in English? I don't mean pronunciations that are hard to guess, just words that are physically hard to pronounce.

The hardest words in English to pronounce depends on your native language. I have found that people whose first language is Chinese have great difficulty pronouncing "brother" exactly correctly. Chinese doesn't have consonant clusters, the 'th' sound is difficult for people from many language backgrounds, and most Chinese speakers (at least in Taiwan) do not pay attention to the difference in the vowels of 'rub' and 'rob.' I can spend a lot of class time frustrating everybody with 'rubber' and 'robber.' I have seen 'antidisestablishmentarianism' on lists of difficult English words to pronounce, but my students don't have any trouble with it. Just because a word is long doesn't mean it's hard to pronounce. Tayal is an aboriginal language in Taiwan. I think Tayal pronunciation, especially Squliq dialect, sounds great but is difficult to pronounce. It took me months to pronounce mqalux (black) and rxyan (mountains) approximately correctly. But my Tayal friends find it almost impossible to pronounce English words like "swim" or "first." So the difficulty probably depends a lot on what you have spoken since you were a kid.