Mastercard probably has the highest acceptance, very close second is VISA. Amex is a distant third, and Discover is accepted in Walmart and Esso.
If you're not into churning and want simple rather than a whole extra redemption system with restrictions, the CapOne Quicksilver is probably your best bet. They are known to check all credit bureaus rather than the usual one, in case that matters to you.
They do a chip version, you should probably get that if you can.