I'd say Java, except I don't like Java and getting native-looking UI seems to require rebuilding it for each platform, taking away it's one merit of "running th

I'd say Java, except I don't like Java and getting native-looking UI seems to require rebuilding it for each platform, taking away it's one merit of "running the same jar everywhere". Qt is relatively easy to cross-compile for at least linux and windows, and if you're mac-native, it might be easy to build for all three platforms