With regards to location, the population in Canada and Australia is much more concentrated in a handful of cities than it is in the US (I’d guess that the UK is somewhere in the middle). Hence, the exchange universities there are more likely to be in a city comparable to Singapore.
Outside of the big names, the US has some very good universities, but not all of them are in the main cities and a fair number of them are in college towns (rural areas or smaller cities with not much there besides the school).
For the UK, London is probably on a short list of global cities for which there are very few peers. I’m not sure what it is like elsewhere in the UK.
The rest of Europe and Asia all have their own language/cultural nuances.
For a semester or year, I also don’t think it matters very much. The underlying experience will be similar. Hence, target where you like, but ultimately be happy with what you can get or what works with respect to courses.
There are probably other considerations if the child is looking at working abroad for a few years or potentially seeking residency. Socially, culturally, and politically, I think most of the English-speaking countries are fairly similar.