You're posting in Bitcoin Discussion, so I assume you're talking about buying Bitcoin for the cheapest price...

I'm not sure that's possible since BTC is probably the one coin that's got a solid price across most exchanges.

Plus, it's hard to buy BTC with fiat on most exchanges, because there are only a select few that allow you to deposit fiat: Kraken, Coinbase, Korean exchanges, Japanese exchanges.

If you're looking to buy bitcoin via paypal or some trading like that, localbitcoins is a good bet.