To achieve maximum flavour is it better to cut or tear basil leaves?
Tearing is for artistic effect

Some people will claim tearing does not damage the cell of the plant as much as cutting, a simple look under a basic microscope will show you otherwise. Not sure how this would affect basil in particular. What dish are you preparing?

If you want more basil flavour and smell, cut it more finely, or bruise it (back of knife or rolling pin) to release more oils just before serving