Cocos2d-X might be a better option.

Phaser[0] is Javascript based (you can also use TypeScript), and has a ton of documentation and community.

[0] phaser.io

p5.js and p5.play where what I used to make a SHMUP.

https://nortyspock.github.io/Viktor/

Before that I made asteroids. https://nortyspock.github.io/asteroids-p5/

Godot[1] is free and open source, similar to Unity, builds to mobile, desktop and web. It supports 2D and 3D, as well as VR through some addons. It has a built in python-like scripting language called GDScript that's easy to pick up and understand, and you can also use C#, C++ or many other languages through bindings (even Javascript[2]).

It's a really impressive effort for an open source project. There's a channel on YouTube called KidsCanCode[3] (not just for kids!) which has a load of project-based tutorials to build lots of types of games.

[1] https://godotengine.org/

[2] https://github.com/Geequlim/ECMAScript

[3] https://www.youtube.com/channel/UCNaPQ5uLX5iIEHUCLmfAgKg/pla...

The replies talk about engines, but you asked about which Flash game would be good to convert, right? Or did I misunderstand?

Not flash games, but these are ones that I have done in the past that were good for learning:

For puzzle games, Set is a good one that will let you practice the UI aspect and recording state, as well as scoring. The PDFs here have the rules, https://www.setgame.com/set/puzzle_rules, and you can play the game at https://www.setgame.com/set/puzzle.

For real-time games, Pong is a great first project and can be followed up by a top-down 2D game (probably a shooter is the most common one). For these you'll need to use Canvas.

Unless you're doing something unusual, the game mechanics are usually not the hard part. A lot more work has to go into polishing it and making a finished product.

I would suggest staying away from Unity in the beginning, it adds a lot of complexity and you'll be able to get started much faster with a bare canvas (which you can easily learn to draw on, and create a game loop with setTimeout or requestAnimationFrame) or React project.