Well, very good question. But there are something which we need to know before answering this question.
The guy whom you are thinking your best friend, is really a best friend of you? Whom we call best friend? The friends who always come to help in need, stay on our side in bad days, spend good times and etc etc. Mainly he won't leave you alone in danger and will always try hard to help you. If you have a best friend like this then choosing one best friend like this is the best option.
In case of friend, i would like to say- In our day to day life we make many friends. But when we are in a need, we don't find them beside us. So what is the benefit of making this kindda friend. So, leaving these type of friends is far better.
By the way no one will love/like to lose a friend. But if there is such a situation choosing the best one is the best option for you.