Well, there are a few things to look at to find the best exchange:
1. What you need? Some sites allow to trade coins, others only buy or sell.
2. Where are you located? Not all exchanges work everywhere and it’s better to choose a reliable international site or your local merchant with clear rules.
3. Can you trust it? Good exchanges store currency on cold wallets, offer 2FA and 24/7 support.
4. What coins you want? Different sites support different currencies, be sure that the chosen one supports all desired coins.
5. How much you are ready to pay? It’s about accepted payments and fees.
There are a lot of options, which are really reliable (gdax, kraken, coinmama, binance). As for me, I prefer cex as it works with cards. Though fees are quite high so you better check if you can do with them -
https://www.katiewager.org/cexio-review.