You can try Coinbase which is great for beginners or Poloniex which is better for the traders which are looking for the margin trades. There are also Kraken (It is offering the complete set of features which is attracting a lot of traders), Bittrex (I think that there are listed more altcoins then on any other exchange site, personally didn't register there) and Binance (Low fees are attracting).
There are also other exchanges and not all of them are suitable for the same type of the traders.
Try to google it a bit more and search their cons also so you can find the most suitable exchange that will benefit you!