If you define "strong fiat" by: a low
public,
external, private and commerial debt, you'll end up with a "third-world" looking list.
If you define "strong fiat" by: used by a lot of people, it will be the USD.
Obviously, both methods fail. We might consider a fiat currency as strong if it has a stable percentage of money growth, which ideally should be 0.
I mean as far as purchasing power goes, If you wanted to cash out some Bitcoin what would be the best currency to cash out in?