Purely from the standpoint of making GUI apps in macOS/iOS, Swift is almost certainly the best choice. All of Apple’s UI frameworks are written in Swift (technically often Objective-C, but with Swift in mind), and designed to be used from Swift. It’s kind of possible to do this in C++ using Objective-C++, but nearly all of the UI code is going to be Objective-C anyways (Objective-C is the language that used to be the default on Apple platforms, but was replaced by Swift). It’s also certainly possible to use libraries for other languages that wrap this functionality, but these often can be missing features and/or be harder to work with. Additionally when looking for help, you’re likely to find much more support out there for the native frameworks since that’s what most people are using.