It depends on your needs. For example, Binance is a good exchange with big volume which is good for daytrading but the fee is a bit expensive (except using BNB) and not allowing you to trade with small number of altcoins. Bittrex also has a big volume with low fees and allowing you to trade with small number but you need to pass KYC in order to be able to withdraw you altcoins and the support is a little bit late. For the platform/UI, i can't say which one is better as it depends on the people itself. That's all based on my experience.