The most successful modern prog music nation?

Whatever you can tell. 

It was successful back in its heyday, but if you're talking about nowadays it's underground af.

Anyway I'd say Central Europe and the good ol' USA if we're talking about dope Avant-Prog.

Record Connection in Ephrata, Pennsylvania, currently has had one for at least the last 20 years (I discovered it in 2003). I haven't visited it in over a decade because online stores are cheaper and it saves on gas. The owner used to sell prog CDs at the local Rites of Spring festival when it was held in Gettysburg. I never made it to the festival. Wish I had. They did have some nice acts show up.

EDIT: Actually I was in that area a month ago but only because there are a lot of antique malls along PA state route 272 that were all having 20% off sales. I didn't have the time to visit Record Connection and was low on funds because I bought a 1973 Topps Mike Schmidt rookie card.

^I don't think I've ever seen a prog section in USA or the UK (or more specifically in New York or London. I did come across one in either Glasgow, Edinburgh or Dublin). But in my experience, progsections in record stores I've visited all over Europe is not particularly unusual at all. Progfans are passionate about their music and among those who seriously buys and collect albums + are willing to spend a lot of money for music. It would be bad for business not to make yourself attractive for this group of costumers. Record store owners/workers are often progfans themselves. I often see the relevant genres separated into something like: Progressive Rock, Jazzrock, (Progressive) Electronic, Experimental and surprisingly often: Krautrock. I'd say all these sections are just as common - or more than New Wave/Post-Punk.

+there's been several progsections at every single record fair I've ever been to. But that's perhaps less surprising.

In NYC there were two CD stores that had prog sections around the late 1990s into the early 2000s: Outer Music, which I believe was around the corner from The Strand bookstore. I remember they had a lot of Eloy CDs there. The other store, which I can't remember the name of, focused more on psych stuff and was about 3-4 blocks east of The Strand and was in the basement of a house. I wish I could remember what that place was called.

I haven't been to Manhattan or Toronto in the last 20 years.

Why are you answering for someone else?!