Cross-Platform GUI

I've seen this one come up before, and have seen some similar libraries mentioned. I would like to know what is the best cross-platform GUI builder for C/C++. I have quite a bit of experience now with the Win32 API in C, but would like to move onto using the power of C++, but feel that it wouldn't be quite right to simply use the (mostly) non-objective, C-targeted Win32 API or the bloating MFC.
Primarly I'm looking for platform-indepence, good documenation, a rich library and to a large extent, native resemblance.
I am right in looking for a widget toolkit as a solution? So far, I've researched GTK+, GTKmm, Qt, MFC, FLTK, wxWidgets and a few others from Widget Toolkits. What I found so far is that wxWidgets seems to meet all requirements the best, so I'm wondering if anyone has any pros/cons to add to this, or suggestions for other toolkits libraries.

MFC ain't cross-platform, so you can safely forget about it.

Qt, GTK and wxWidgets all have quite mature GUI designers. You'll have to try them out to find what you like best. The thing about GUI libraries and designers is that from 3 developers, you'll likely get 4 opinions.

http://www.ultimatepp.org/
is another option. it ships also with an gui designer and a separate ide, too. check out the comparision map on the website

If you want a list of even very obscure GUI libraries, try this: http://www.atai.org/guitool/

Thanks for your help guys, I think I'm going to go with wxWidgets as it seems to be the best at all the above (GTK/Qt came close, but wxWidgets had the edge on native emulation).