If you are talking about speed , then it should be Binance.com
Binance is a cool exchange, but if you look at volumes (
https://neironix.io/exchanges), then it already has serious competitors, but of course I like Binances technological development, the exchange is very actively developing and popularizing itself around the world. For margin trading, I prefer the Bitfinex interface and orders for margin trading to be made very high quality, somehow all user-friendlings for margin trading are somehow not right on the finances.