If you only want to trade the big coins (Bitcoin, Bitcoin Cash, Etherum, and Litecoin) then I would always say Coinbase. It is easy to use and they have amazing security measures. To my knowledge they have not been hacked. It is also good if you are trading fiat money for coins instead of coins for coins.
If you want a variety I would say Binance. It has a ton of options, it is easy to use, and they add new coins fairly often.
If you only are trading cryptonight coins I will always recommend altex or trade ogre.
Never use cryptopia they are unreliable and will probably fail and crash in the next little while.