I use several trading platforms. Only choose top platforms for safety.

https://site-guides.com/tag/cryptocurrency-exchange/

The above link has a list of top cryptocurrency exchanges. Research these top websites and choose the one you need. I use Bittrex and Poloniex for alt coin trading. I am also using Bitfinex to buy IOTA, as IOTA can only be bought at Bitfinex. When I need to withdraw fiat, I use Quadrigacx for Canadian dollars. So there is probably no single exchange that can meet all my requirement.