NZ - 95.37% native Anglo speakers
Aus - 72.0% native Anglo speakers
Can - 58.1% native Anglo speakers
USA - 78.0% native Anglo speakers
The answer is New Zealand by far, followed by USA and Australia roughly close together, and dead last being Canada. Makes sense, considering so much of Canada is Francophone and immigrant languages like Spanish and Chinese are widely spoken throughout America and Australia while being almost completely absent in NZ. The only real non-English linguistic influence on New Zealand is Maori, spoken by about 4% of the population.
The 95.37% statistic is not about native language and is from a question where people could choose multiple options (I believe it just asked about languages spoken overall) which means the total add up to more than 100, therefore the comparison is faulty. In fact I'm fairly certain NZ has a higher proportion of Chinese speakers than the USA.
And for that matter, there a lot of immigrant languages being spoken in the UK as well.
If we're not talking about native language, then 98% of Icelanders knows English, so it must be super Anglo.