you did not write for what trading/buying ? most people will probably say that binance and coinbase . EDIT tiny overview: https://nomics.com/exchanges
They are cheap for trading but expensive for crypto withdrawals. A good offramp though if you are pulling out cash.
binance and kucoin are great since they have good liquidity and API is quite stable so it's perfect for scalping.
I use Kucoin, had no issues with it. I used binance before, tons of problem, esp. when you want to withdraw when the market is volatile.