In my experience, in terms of overall quantity of spending, US viewers are the best bet.
I'd say pair UK viewers with kinkier models when possible - it's like Brits simultaneously have all the shame and no shame
I've never ever been upset with having an Australian in my room, whether or not he spent any money.
Canadians... um... maybe they hate me, or maybe they're so polite that I never even notice that they're there, or they just never share their location, but I wish I (knowingly) had more Canadian guys.
I rarely get customers that aren't English speakers, or at least pretty fluent in English as a second language. I think that's because I talk a lot, and it might hard for them to follow if they are struggling with a language barrier. So maybe it's better to direct non-English-speaking viewers to rooms that are more oriented to sexy dancing or other less verbal activities.
Avoid marketing to: countries where porn is illegal, countries that don't have compatible credit cards or other means to spend money on our sites, countries where the typical income is low enough that paying cam model rates would be prohibitive. I also suspect that certain European countries have thriving cam sites devoted to their own language, so they aren't very active on more global/American sites.
Stereotypically, I think we tend to get a lot of irritation and minimal spending from cultures where the expectations of female sexuality and sexual agency are significantly different than Western standards.
Whatever country produced "bitch lasagna"... I want to say avoid it... but I'd also like to know wtf that means, so...