The Queen of England speaks best (and most understandably). There is an expression Queen’s English, and it means “reference English”. Some Oxford and Cambridge professors speak no less clearly: they are carriers of exemplary, academic English pronunciation. All other native speakers of English have some deviations from the normative pronunciation english to russian.
When talking about English accents, they usually mention the difference between British and American English. Of course, there is a difference in pronunciation between these two peoples, but it is not so great. Anyone who has learned to understand Queen’s English will get used to the peculiarities of American pronunciation in a couple of dozen hours of communication. However, there are English accents that deviate much further from standard pronunciation than American English.
The Scots, for example, speak their own English. They seem to speak English, but it seems that they have a hot potato in their mouth, and therefore they cannot pronounce words normally.
In India, English is also the official language, but understanding Indians can be even more difficult than understanding Scots – if you have mastered only standard English pronunciation english to french.
Australians and New Zealanders also have their own accents, and with a classical education, it will not be easy to understand them, especially the latter. And in England itself, speech in the countryside differs markedly from the speech of city dwellers. The accents in this country also vary from city to city and region to region. Remember Pygmalion by Bernard Shaw: Professor Higgins could tell by his accent what part of London the person talking to him lived in!
Thus, mastering the skills of understanding sounding English speech in full is, perhaps, an impossible task. But this is not required from a practical point of view. When you learn to understand standard English, you can quickly get used to one of the English accents if you need to.