Choosing from the wikipedia list of English Dialects, which dialects are considered the easiest for learners? And related, which dialects and considered the hardest for learners? For example Received Pronunciation would likely be easier for a learner to understand than Cockney.
This question is meant to be focused on spoken English, the pronunciation and enunciation best suited for learners. Factors to take into account include, but are not limited to:
Will knowing the dialect make other dialects of English easier or harder to comprehend? (e.g. London Cockney is one example of a dialect which is notorious for being more difficult for English speakers from other areas to understand)
Does the dialect rely on heavy usage of slang specific to that dialect?
Is the pronunciation of each phonic sound distinctive? (can easily hear the difference between sounds)
Is the pronunciation standardized? (not localized to small region, able to use the same IPA for similar spellings)
Are the phonemes of the learners native language more compatible with the dialect?
Pace of Speech
Is the average spoken wpm too rapid to be easily understood?
Are words often "mushed" together? (e.g. "cannaehavit" instead of "can I have it")
If other reasons/factors not listed here influence the answer, please include them with the answer and I will revise this list of influential factors.