Let me be frank. I don't think it matters, and here's why. I have known dozens of professors, scientists, colleagues. I've known people who left Austria in World War II and had been in the United States for _forty years_ when I knew them. Many of them were much more than just "fluent." In some cases, they could speak, not just sentences, but whole paragraphs of clear, well-phrased, grammatical English--better than mine. They could understand jokes, colloquialisms, and slang. They could understand rapidly-spoken English. But they retained their foreign accents. In some cases, heavy foreign accents. In others, slight and "charming" foreign accents. When you listen to someone with a foreign accent, what you hear is their foreign accent. Unless you are paying close attention, or unless there is some specific giveaway in vocabulary or phrase choice, you hardly know whether they are speaking "U.S. English with a Russian accent" or "British English with a Russian accent." You just hear "English with a Russian accent." (I know there are exceptions, foreign speakers who have such a fine "ear" that they have learned to speak "accent-free" English, and obviously I might have met them and not known it. And if you have a serious goal of speaking, not just fluent English, not just excellent English, but perfect accent-free English, then I guess it does matter).

"What is the best English accent to learn?" I think that there is an answer to this question, but it depends in part on the following question... Where do you plan to use spoken English in the future? In Algeria, where you apparently live, and elsewhere in North Africa/the Middle East? The U.K? European countries in addition to the U.K. (e.g. Germany, Denmark, Netherlands)? Canada? The U.S.? Australia? New Zealand? English-speaking Africa? India? If you can answer this question, I'll be delighted to make a suggestion.

It is your choice. Be aware that some words and idioms in BE and AE have different meanings -- not a huge amount but some. If you like to listen to American movies and music, then perhaps American English would be better because you may want to be able to understand the AE meanings of the words. I have had some foreign students tell me that they can understand my AE better than other teachers' BE. I don't know how true this is, but that's what some of my students have told me.

I don't think there is a 'best' accent to learn in English. I think it's up to what you feel more comfortable with and where you get the better practice in :)

It really doesn't matter that much. I will say though if you're learning British English a British accent makes more sense and if you're learning American English an American accent makes more sense and so on. Even if you mix them though its fine, English speakers can generally understand other English speakers just fine .