It also depends on your location. What is your country? Coinbase is the most popular internationally, I guess, if you're talking about buying BTC using fiat money. You can google the available exchange in your country. If you are Asian, some countries in Asia has so many local exchange that you can use.
But if you already have BTC and just looking for the right exchange to buy or trade with the cheapest price, I recommend Bit-Z. It's not as cheap as the cheapest but they have lower fees. . Trust me, Fees are eating your money.
Yes, country can play main role in determining the exchange you buy crypto currencies from. There are exchanges in many countries from where you can even buy altcoins with fiat money by either payment from credit or debit card or by payment from bank account.
I am from country India and there are exchanges here such as zebpay, wazirx and koinex that offer altcoins and bitcoins in exchanges for fiat money.