What is the cheapest way to buy bitcoin?

I'm curious to know what the most cost-effective method is for purchasing bitcoin. With the numerous options available, from centralized exchanges to peer-to-peer platforms, it's challenging to determine which route offers the best bang for my buck. Could you elaborate on the cheapest way to acquire bitcoin, considering transaction fees, exchange rates, and any other potential hidden costs? Additionally, I'd appreciate any insights into potential risks associated with the various methods and how to mitigate them. Your guidance would be invaluable in my quest for cost-efficient Bitcoin purchases.