'What is the best qualification to teach English as a Second Language?' is a very good question, and one which few if any armchair critics can answer without exposing themselves as 'know nothings'.
If anyone answered 'a primary or secondary school home-country qualification' then you need to realise that teaching native-speakers aged 5-18, never mind 50 year-old bankers, is a completely different kettle of fish.
I've worked with teachers who meet that criterion btw, and that is not their answer.
Yes, even semi-literate TD know-it-alls like Jack meoff can have a go.