When comparing libGDX vs Monkey X, the Slant community recommends libGDX for most people. In the question“What are the best 2D game engines?” libGDX is ranked 14th while Monkey X is ranked 27th. The most important reason people chose libGDX is:
Lots of references, tutorials and open source code to learn from.
Specs
Ranked in these QuestionsQuestion Ranking
Pros
Pro Lots of resources to learn from
Lots of references, tutorials and open source code to learn from.

Pro Great performance across devices
While it may not always have the best performance for a particular device or system, it's one of the few engines which consistently performs very well across different platforms.
Pro Fantastic UI toolkit (scene2d)
scene2d is a 2D scene graph, scene2d.ui is a UI toolkit on top of that. It enables very complex mobile and desktop UIs.
Pro Can use any JVM language
Since it's built with Java and runs on the JVM, any language that compiles to Java bytecode can be used to develop games with libGDX.

Pro Uses Box2D
A Java port of the Box2D physics engine is included in libGDX.

Pro Allows you to migrate from 2D to 3D within same framework
LibGDX supports both 2D and 3D game development. Allowing developers to migrate from 2D to 3D with ease without having to switch the engine they are using or having to learn a new API.
Pro Doesn't force a specific design
Unlike some engines, which only allow you to make your game in a few specific ways, libGDX allows you to design any type of game you wish as it is extremely customisable.

Pro Active and helpful community
The libGDX community, in the official libGDX forum is extremely helpful and approachable for any kind of question regardless of the its quality or difficulty.
The forums themselves are a very helpful resource for any issue or guide simply by searching past posts in there.
In addition to the forums, there's also the official #libgdx IRC channel on Freenode.
Pro Interfacing with platform specific code
Sometimes it is necessary to access platform specific APIs, e.g., adding advertisement services or leaderboard functionality provided by frameworks.
Pro Free, open source & permissive license
libGDX uses the Apache License 2.0.
Not only is libGDX free and open source but also it's license gives you a lot of power over the engine. As long as you provide a copy of the license, give credit, do not hold devs liable and do not use libGDX logo in any engine forks you can do pretty much anything you want.
Pro Kotlin support
It is written in Java so you can easily make games using Kotlin and Ktx project will help you get all advantages of this language.
Pro Very easy to customize
Pro Constantly improved
LibGDX itself is pretty mature, and get updates not quite frequently, but various libraries for it are actively updated.
Pro Across platform support
You can write once and run anywhere (Android, IOS, Desktop, Browser) also lately came with lib to deal with VR.
Pro Support for 3rd party tools
libGDX has built in support for many 3rd party tools, including (but not limited to) Bullet Physics, Box2DLights and the well-praised Tiled Map Editor.
Pro Lots of tutorials to get you going
Even though the official documentation might be lacking, there are many tutorials on YouTube for libGDX.
Pro Excellent font rendering support
Very good tools for rendering fonts.
Pro Similar to the Microsoft XNA framework
Old XNA users may like libGDX since it's API is actually very similar to XNA's.
Pro Focusing on object pool patterns, to control memory without pointers
Unlike Unity or other engine, it allows to optimize a language that uses garbage collector when using patterns of objects you can control the use of memory without needing a language like C / C ++, getting the same speed in a more productive language.

Pro Native code support
Developers can make native calls directly from Monkey code. This allows access to any native functionality and platform-specific features.
Pro Native module support
You are not restricted only to the modules you get from the official release. You can build your own stuff. Even build your own "app" module. It feels limitless. In comparison with other cross platform solutions, you actually get the translated source code and you can play with it if you want.
Pro Built-in modules for quickly building games
Monkey X has a selection of great built-in modules.
- Graphics
- Audio
- Input
- Data and Filesystems
- Networking
- Math
- Text and Strings
- Collections
- Online Services
Pro You can create custom targets
The language gives you the possibility to build your own targets. You are not limited to the targets officially supported.
Pro Low cost license fee
All target platforms for a fair one off license fee.
Pro Easy to learn
With it's Object orientation and clean syntax its a brilliant language to learn if you have never done any programming before and yet still has all the power it needs to make full games and apps.
Pro Uses a great, easy to learn language
Monkey X uses a custom programming language (called Monkey) for all its scripting needs. Monkey is rather easy to learn, it's object-oriented which will help most programmers with understanding it. It's also statically typed and uses a garbage collector, helping to avoid manual memory management.
Pro Partly open-source
The entirety of the base-language itself is open source. Commercial modules such as Mojo for non-free platforms cost a one-time fee. Though Mojo is not free for all targets, the targets for these platforms are, meaning it is possible to implement other frameworks for these targets.
The Desktop (GLFW and C++ based) and HTML5 implementations of Mojo are currently free and open source.
The language's development is completely public, and is managed via GitHub.
Pro Free HTML5 and Desktop (GLFW) target platform
The free version of Monkey X lets you compile to HTML5 or Desktop (GLFW). Other platforms such as iOS, Android (and OUYA), XNA, Flash and Windows 8 (Phone) require the paid version of Monkey X Pro.
Pro Cross-platform
Monkey X is a cross-platform game engine. It allows developers to run the same code on multiple platforms, including iOS, Android, Windows Mobile, Flash, Windows, Mac OS and Linux.
The development environment supports Windows, OS X, and Linux.

Pro Not running in its own VM
Unlike other multi-platform engines (Unity3D, Corona, etc), Monkey-X games do not run explicitly in their own virtual machines. Your code is translated into the native languages of each target platform, and then compiled as a native executable. However, just as native games, on platforms such as Android (Currently), and HTML5, games will be ran through the targeted platform's usual VM(s). That being said, you won't be dealing with a proprietary virtual machine, so you won't experience any real overhead when compared to a native game.

Pro Made by the Mark Sibly Factor
The Mark Sibly Factor denotes that a programming language will be easy to learn, fun to learn and allow any age group ( within Cognitive Reason ) to program games and great games. The Mark Sibly Factor denotes also that the games programming language you purchase will be backed by decades of compiler programming experience, game making tool programming and finally a Game Programming Language that kicks Ass.
Pro Free for commercial releases
With the free version of Monkey you are still able to create commercial HTML5 and Desktop games.

Pro Many community modules available
The community has created essential modules:
- Spine for animations
- Box2D and Chipmunk for physics
- Game frameworks such as Ignition, fantomEngine, and Flixel
- Diddy for lots of extra functionality
- FontMachine for custom bitmap fonts
- MiniB3D for 3D gaming
- And several others...

Pro Drawing Commands
Drawing commands are easier to read and edit than is manipulating scene graphs. You can immediately see in your code what's going to be drawn and in what order. Transformations are a no-brainer too.
Pro Lots of great examples
Monkey X includes over 50 examples ranging from complete sample games to demos of single features.
Pro Object oriented programming
MonkeyX is an object-oriented dialect of BASIC that's easy, clean and powerful.
Cons
Con Not starter friendly
Even default applications fail to load in Android. It doesn't have any documentation on errors either.
Con Documentation could be better
Other than a brief installation / getting started overview, libGDX's documentation consists of an official wiki with several incomplete pages, and automated Javadocs. The community recognizes these shortcomings, and new users are encouraged to ask for help.

Con The project slowed down in development
The number of active developers has decreased, many open issues and pull requests.
Con Build system is clunky
LibGDX uses Gradle, which is very demanding of memory and makes IDEs freeze on anything if your computer is not powerful enough.

Con Does not create compiled code
LibGDX runs entirely on Java and does not create executable binaries.
Con A bit difficult to use
This engine is not well put together. Is made from various free modules each with their own peculiarities. At times it feels you need to learn a couple of libraries rather than just one. Is not an engine for beginners as it requires coding. Lots of coding. You need to be intermediate to advanced in Java to develop in LibGDX.

Con You'll have to learn a new programming language
Even though Monkey is rather easy to learn and borrows a lot of concepts from more popular languages, having to learn a new language develop games is a lot of friction for people that already know how to program in other languages..

Con The documentation is not very thorough
The documentation contains a reasonably detailed language overview, and a somewhat-generated list of the included modules, classes, and methods. Module descriptions are rather lax, but usually present. Method descriptions tend to be short, and a majority of them contain no usage snippets; most parameters have very minimal descriptions. And there are no community collaboration features to help improve it, besides GitHub.
Con The included IDE is poor
Although better IDE'S are availabe for a price, the default one is bare bones and lacks functionality
Con No real asset store
Untangling how to keep assets in the ".data" requires attention and a filenaming convention.
Con limited OS export targets with free version
Free version only targets Desktop (macOS, Windows, Linux) and HTML5, not mobile.
