As some member said, BTCPay is best option, the only downside is you need to setup BTCPay server by yourself. If you don't have ability to setup it by yourself, you can use third-party hosting (
https://docs.btcpayserver.org/ThirdPartyHosting/), but you must trust them.
You can checkout Bitpay, I'm not certain if it is integrated with WordPress, but a quick search around suggests so. You would do your research on this.
It's open source and operates on an API. It also works with electrum wallets and is compatible with segwit addresses.
OP ask for best bitcoin payment gateway, not worst bitcoin payment gateway.