What tools/languages are the easiest for you to build side projects in?

Really depends on the side project. I'd pick C if writing low-level/kernel code, OCaml for language-processing tools, Python for scripting/ML/Algorithms. Go for distributed systems. It'll also depend on available libraries. Occasionally C++ if I need speed + data-structures. But most of the time, my side projects are just a pretext to learn something new, so I would happily pick up a new language.

As for the tools, I use VSCode that works well with all languages, and git (github/gitlab, probably gitlab). Then VMs or docker. And unix shell/bash.

I make games as a hobby, and while I'm often paid to write C++, I'll use plain JavaScript with various libraries (Phaser, three.js, ink, etc.) as iterating is very quick and distributing is as well. I don't think it's a good favourite for everyone, but it helps me find the joy in making things.

The biggest boon is copy/pasting a GitHub pages link to a friend and quickly asking for their feedback since everything runs in Chrome.

Clojure / ClojureScript -- one of my favorite things to do is prototype new features in a fresh clojurescript template based on Figwheel

cop-out answer: whatever you are most comfortable in OR whatever you most want to learn to get comfortable in.

long answer: depends on the question. if you mean you as in the Royal You, hard to say, hence cop-out answer

if you mean you as in me personally, then I would say it further depends.

Windows desktop app? Visual Basic 6 with forms designer.

Linux desktop app? I've never done it but have standing plans to find a not-terrible IDE and learn QML.

Android/IOS mobile app? Xamarin Forms with .net core

web app? SAFE template with .net core