Javascript, Android, Python as well as PHP Programming language is the best for web development and mobile development.

C# with ASP.Net Core for Web and Xamarin for everything else.

One language, one code to rule them all, its performatic and robust

JavaScript.

React Native for mobile, Electron for desktop, React.js with SSR running on Node.js

Reason: simple to set up, pretty performant, fun to write (in my opinion)

I have tried Dart, It's something like the best of JavaScript and Java in one package. Best of both world, faster compilation time and easy to learn.
Go to dart.dev, everything is perfectly documented and you won't need any tutorial videos if you're familiar with Java.
With Flutter and Hummingbird (flutter on the web), you can dive into front-end development on both mobile and web platform very easily. Learning dart is the best thing you can right now.

Dart with flutter.

For me, my only choice for desktop, mobile, embedded and now even web is always Qt/Qml :), Qt is a c++ Framework available for large scale systems and embedded and now available to web using Web-Assembly, so one App Multi-platform :)

Using Qt Also allow me to extend my app in certain platform using their native toolkit, so i can write one app in Qt then add a java plugin for android or swift code to support certain feature in ios.

Even thought Qt is a C++ Framework I can write my app using only JavaScript or Python :)

-- Learn Qt here : https://qmlbook.github.io/

-- Qt is a free open source toolkit :) do not get back because the website is saying you have to buy it :)

Mobile development: Flutter Web Application : ReactJS.

Reason: Learning curve is fast.

For playing around? I think, Rust is making some nice progress on the web front, and there are quite a few projects for mobile, too (fusing Flutter with Rust, for example). I don't know how far I would get, but that's what I'd love to do.

For freelancing purposes? It's unlikely that I'd create anything really big, so I would most likely go for a hybrid app on top of Cordova - if possible. WebDev still offers some of the best UI tooling around, and I know that stuff inside out. However, if that's not an option, maybe Object-Pascal or even Dart (Flutter). I wouldn't go as far as to use the native languages - too much trouble for freelancing imho. For web, NodeJS or PHP.

For corporate purposes? Preferably native development (Kotlin / Swift), but maybe also Dart (Flutter). On the web side, I'd go NodeJS or Kotlin. Rust might be nice, but is missing stable releases and SLA contracts...

If its mobile then I would go for Dart - Flutter (Mobile Application) and Kotlin is better option for both web and Mobile

There are quite some languages I like more than Java, but the one that's usually applicable in the context where Java is (JVM, Android) is Kotlin.

Kotlin runs on the same platform and is somewhat similar in philosophy, and can call and be called by Java code. Yet it solves a lot of the issues in Java.