The best place to buy/sell BTC and ALT coins in my opnion is KuCoin and Binance
KuCoin has more pairs then binance, and binance is much more optimized the kucoin and is the leading crypto exchange.
Now if your looking to trade futures, (long and shorting) i highly recommend ByBit.
You can PM since i am a partner with ByBit you can receive 10$ and percentage off fees using our partner link. Im not sure if im allowed to post my referral link here.