I don't think Original 6 jerseys are that much better than some of the better, newer jerseys in the NHL. It's just the fact that they've been relatively unchanged for 60-100 years gives it a sense of superiority. Detroit and Toronto especially. If they didn't exist and a new team from Las Vegas or Seattle came in with plain red/blue and white uniforms, people would say "They're ok, but kinda boring".
If I am being as objective as possible, I would say Chicago home and away, Boston home, Montreal home are among the best in the league. The Rangers home and away are very nice too, top 3rd in the league for sure. Detroit and Toronto are kind plain, not a huge fan. Montreal and Boston's away are nothing special either.
But they are still better than a lot around the league that have ugly components.