Binance is the most popular. But it's not the best, IMHO.
As for me, I'm using exchange services (usually changenow or any other similar) to swap or buy crypto.
I also used changenow and I liked this service. Some pairs look much better than on other exchangers, and I use the fixed rate they offer. I stopped using bittrex after I had problems with my account, so I use changenow without registering.