What does 'popular' mean? Number of people that speak it? Number of countries that speak it? The USA has a large population, so probably wins on total speakers. The size of the British Empire probably left a legacy of many countries that speak with more of a British accent. In terms of what people that are learning want to learn - it probably depends who they want to talk to most, and where they want to visit. If you wanted to get a job in the USA, you might want to cultivate a US accent to get accepted more easily. But more importantly - it's just an accent. And as a non-native speaker, your own accent will be much stronger than any accent you try to cultivate. To me - your accent will sound more Arabic than it would sound British or American, unless you spend a lot of time in that country. And both accents are mutually understandable. You will be understood whichever accent you try to emulate.

Maybe you mean which is more popular, American English or British English?

Depends where you live ... I would imagine american is more popular in america, and british is more popular in britain.