Obviously BitPay is the best payment gateway and major/big company such as Steam use bitpay, but as far as i know they only act as gateway for bitcoin payment.
However, i don't know any recommendation for altcoin payment gateway since i don't have/use altcoin at this moment. But, you could check what altcoin payment gateway that other services use for some reference.
As mentioned bitpay is the best of all in terms of service and the processing method as a payment portal. If you go through websites that accept Bitcoin, you could find most of the websites accepting Bitcoin through bitpay.