As we speak, though they've had problems with downtimes in the past, BitMEX is still the best in terms of reputation and liquidity. Well, probably unless you're from the United States(if I remember correctly US residents aren't allowed to use BitMEX).
Though of course, BitMEX is a huge ass exchange hence it's a huge ass target for hackers so don't unnecessarily leave funds there.
Yes, we all know bitmex is one of the largest margin trading exchanges in the crypto market, followed by bitfinex and Binance exchange. But for a new member who is starting to trade, choose small exchanges with lots of bonuses. they will make many mistakes and bonuses will help them to make a profit so as not to make too much losses. I think Snapex is the best choice. When the trading volume is 100k $, you will receive 100 USDT.