Americans by far. And it's never been close.
I realized this when I Worked in a phone store for a couple of months in the early 2000s. Americans live in a consumer friendly environment in the United States because businesses have so much competition they have to bend over backwards to attract and keep customers. As a result the typical American consumer is extremely demanding a with high expectations and downright spoiled/high maintenance.
As I remember my Trini friend who was responding to my American friend that was complaining about the service at a Caribbean restaurant:
CUSTOMER SERVICE IS AN AMERICAN TERM.
I'm sure some of you notice that typical Caribbean restaurant staff ain't all that overly friendly/bend over backwards type. That's because they know their shyt is good and as long as they don't flat out disrespect you personally you'll keep coming back. It's sad but that's the mentality that other nations consumers deal with since they tend to have fewer options. American consumers living in such a large economy always have a plethora of options at their disposal and businesses that are competing for you money know this and therefore act accordingly.
PERSONAL EXPERIENCE:
You have Americans have Businesses in america in the tourism industry catering to foreign tourists and be annoyed that these tourist don't speak English. Yet that same American business owner will go to another country that speaks another language and be upset that businesses there ... don't speak English
You can't make this shyt up
PERSONAL EXPERIENCE 2.0
As I'm typing this I'm in Miami visiting my mom. An hour ago I met up with a Canadian friend of mine at the airport that just came back from a cruise. She was joking about a couple of Americans that were on the cruise that were bytching when they got to Aruba and Curacao. Turns out some fools want to go to other countries and spend American dollars and expect American dollars back for their change. They don't wanna use currency exchange service. They raised hell because they feel like these poor vendors on the docks should have American money available for their customers' convenience.
Again, u can't make this shyt up.