I\'m looking to be able to build GUI applications quickly and painlessly as possible. I\'m competent (though not expert, and have no formal training) in C++, but have never used
Qt is the best option for you. It's the easiest to learn, the most elegant and powerful and it is completely free.
You seem to have experimented with Managed C++. Don't use that, even MS recommends that you only use Managed C++ as glue between C++ and C#.
wxWidgets: This one was a strong contender up to the day when Qt became free for commercial projects. It was always in the shadow of Qt and it is known that the documentation is not very good and the API is not as easy to learn as Qt's. Cross-platform MFC would be a good way to describe it.
C++ Builder: Borland made too many mistakes with C++ Builder and ended up getting out of the dev tools business altogether. It was a good product and I originally learned Windows GUI programming in one of the first versions, but I won't use it any more. There are better options and it is too expensive.