What is best buy in maths?

Could you please clarify what you mean by 'best buy in maths'? Are you referring to the most effective strategy or tool for learning mathematics, or perhaps a specific resource that offers the best value for money in terms of educational materials? If you're seeking advice on how to make the most of your studies in mathematics, there are various methods and resources that can be beneficial. However, the 'best buy' will ultimately depend on your individual learning style, goals, and budget. Could you provide more context or specify what you're looking for so I can offer a more targeted response?