There are many options to choose from. Many comments featured Binance as the best platform for altcoins trading. I agree on it because I was using it for several years by myself, but in as much as Binance experienced a massive security breach on May, which resulted in a theft of 7,000 bitcoins (https://cointelegraph.com/news/hackers-withdraw-7-000-bitcoins-in-binance-crypto-exchange-security-breach), I have some doubts about continuing using it. In general, centralized crypto exchanges are an undesirable place for storing your crypto funds, no matter how secure they are or how secure they claim to be. I think in this situation we can only rely on the platform’s history. For instance, I use CEX: https://cex.io/btc-eur, a crypto exchange that has never been hacked. This fact speaks for its reliability and enhanced security measures. The UI is intuitive and easily navigable, plus there is a mobile app for Android and IOS. As a bonus, in addition to the crypto trading, it supports fiat to crypto pairs.