I'd say a clear speech is the most important of all.
I don't believe British accent is better than American accent,or vice versa. Depending on where you are on earth,being an ESL student,you don't want to study American accent because you love American accent while you are living in a British-dominant country,or try to study British accent and use it in the United States,if u understand what i mean. It's just like you don't want to hear American accent in a Shakespear's play. Just go with the flow. If I were a native speaker,that'd be a different story because,the key is speaking English to let other people understand you and the accent familiar to their ears. ESL students will just get more confused if they want to learn American accent but their English teachers are all British,or vice versa.
I find American accent more easy to understand and accessible because American media is everywhere around us,far more than the British. I don't have much chance to hear about British movies. I see courtesy footages of news from CNN or Reuters on TV more than from British media,say,BBC. I watch both.
As far as BE vs AE,you don't stick to the American date format mm/dd/yyyy while you are in a country where BE is dominant. You'd just cause arguments with others.
All accents are good IF they are used appropriately. It's just my 2 cents.