Users’ preferences vary so it would be hard if not impossible to develop a perfect, universal exchange that would fit everyone.

IMO, in order to create a good crypto exchange, you need to combine the best features of the centralized exchange and decentralized one. This means to make a platform as secure as decentralized crypto exchange, and as easy to use as centralized platforms. Plus add to it the needs of high trading volume and liquidity, anonymity, 24-hour customer support, and lower withdrawal fees. If you succeed in creating a user-friendly and highly secure platform, where the users own private keys, you may achieve high volumes. 

Since there is no such perfect exchange yet, I use CEX. Although it’s a centralized crypto exchange, it is a rather secure and reliable platform that has never been hacked. The exchange is extremely easy to use. It allows fiat to crypto trading as well as crypto to crypto - https://cex.io/xlm-usd. The only disadvantage is tedious registration and verification processes due to the KYC policy.