Other than BitPay what is the top API service to accept BTC, LTC, XMR, and ETH payments? I want to completely integrate it in my website without the user ever leaving my website. Just need to generate an address and display the amount to the user. It must very reliable and fast.
Hands down best alternative for Bitpay is BTCpayserver, it is self-hosted, open-source, no fees and no third parties.
They officially support Bitcoin, but there are many contributions for other altcoins including monero, litecoin, dogecoin, etc.
https://btcpayserver.org/https://docs.btcpayserver.org/FAQ/Altcoin/Another less know open source option is Bitcart, and I think they support all coins you want to be supported:
https://bitcart.ai/