Where is the best option for non-U.S. residents to open a U.S. merchant account?

You might not be able to open a proper US Merchant Account if you are not a US based business with an EIN and some form of a credit check, maybe even a Social Security Number.

However, you can work with PSP (Payment Service Providers) who would work with international companies.

2Checkout is one of the most famous PSP that works with international clients, however, lately the chatter is that they cannot process Mastercard payments (some dispute/issue with Mastercard - this might have been resolved now), otherwise they are decent, albeit slightly on the higher side, which is expected for 3rd party payment providers.

You might want to sift through the Online and Mobile Payments section here on Quora. Quite a few payment service providers have propped up in the recent years and they might be able to work with you.

PSP like Braintree (company) are going International, so you might want to check up with them as well.

There isn't much of a list that is available of PSPs that work internationally, hopefully someone would be compiling one.