See, the great thing is that there's no such place. What's a hard dish? Hard to find ingredients? Difficult prep work? You know, there are dishes where, if you don't, through years of experience, sense the precise moment to perform some step, it fails, and if you don't know that, it's a "hard" recipe, so hard you'll never get it right until you sense the moment.
It's been said, probably with some truth, that some very smart executive chefs test an applicant for a chef position by simply having them prepare an omelet. What does that say about who's the best chef?
The real question is how willing one is to discover the best of wherever you go. I'd pass up a lot of highly touted restaurants for what an old Greek cook cranks out in a dockside place where there's actually some physical danger involved in getting to at night. And I know a Mexican cowboy in Presidio County who can do things with a range-butchered steer that would far outdo what a New York chef could do with the same meat.
You see, talent doesn't always go chasing money and fame. But if I had to pick one region where there was lots of talent, lots of imagination, at work in the kitchen, I might look to Southeast Asia, where the range of ingredients drives some wonderful variety.