I've seen clips of Sesamstrasse and Sesamstraat, and I think they are two of the best international co-productions.
I've been reading up on the various co-productions, and I find it interesting how Sesamstraat had a preschool built in the '80s in an attempt to get kids involved in the show and make it seem less obvious that it
was a show.
I really like Sesamstrasse's theme song, and it is possibly my favorite foreign SS song. I find it hilariously cruel that the opening lyrics translate as "This, this, that?/Who how, what?/Why, why, why?/Those who don't ask stay dumb.". You could never get away with that in an English-speaking show no matter how it's handled, especially these days!
I've seen one episode of Sesame Tree, Northern Ireland's version, and I liked what I saw. Potto is just so silly (I'd like to see him meet Telly, his relative, someday!), and the concept of two Muppets who live in a hollow tree answering kids' questions is a good one.
The Japanese show also interests me, but the only bit I've seen of it was the intro on the Sesame Workshop website.