Sky is the one can be proud of.

Sky Tree is one of which we can be proud of.

What is sky tree?

If you want a simple answer, TokyoCat: Instead of just saying "Sky Tree is one of which we can be proud of," You can say "Skytree is something that we can be proud of." It might would help, as Stu suggested, to add some context (such as "who" is proud of it, and why--Tokyo residents? Japanese? Because it's cool? Because it's the tallest? etc).

"Sky Tree is one of which we can be proud of." yes, perfectly correct, but you need to include context. - On it's own as written it does not make sense. ("one of WHAT that you can be proud of"? ) As part of a longer sentence ("in Japan there are many great examples of architecture, and the Sky Tree is one of which we can be proud of") it's OK, but as a stand-alone statement it doesn't work.