I have always been surprised that UK doesn't seem to have seriously progressed the idea of nominating one or more of its "Garden cities"
Yep same for me, I thought that they were onto a winner with them as they were widely influential and from what I can tell are fairly coherent. I thought there was even scope to tie in the Arts and Crafts movement and some of it's major artworks etc.
I'm not 100% sure on the authenticity of the Garden Cities, I have only visited Welwyn GC and despite the station being a rather bland shopping centre now, the rest of the town felt pretty authentic to me, and the you certainly can feel the space and layout is very planned. I quite liked it, I can't say it was of OUV but I don't really think that the Berlin ones feel that remarkable either, but I think they are worthy of their spot on the list.
In regards to the Berlin Modernism Estates, I wouldn't argue with Hubert's suggestions. I spied Falkenburg and it looked reasonably interesting but that was about all. I visited Carl Legien, I wouldn't say it was that remarkable, but perhaps worth a detour if you are driving there.
Not sure if you have the locations already, but here is a map with the 6 on them