I would say out of those, Paper, mainly due to lack of toolchain complexity or unusual concepts like mixins -- while still being quite capable at more advanced things, including an 'escape hatch' to access raw Minecraft objects in the rare cases where that's needed. It's a very intuitive API. I'd say it's a great introduction to Java.

But other things you should also be looking into:

Skript provides an interesting easy-to-use DSL language for Paper/Spigot servers - https://github.com/SkriptLang/Skript

Scarpet provides an interesting easy-to-use DSL language for Fabric servers - https://github.com/gnembon/fabric-carpet/blob/master/docs/sc...

Opencomputers and Computercraft are mods that adds computers to Minecraft, running Lua. OpenGlasses2 lets you code your own augmented reality glasses in Minecraft, because why not? https://www.curseforge.com/minecraft/mc-mods/opencomputers https://tweaked.cc/

Pneumaticcraft, Psi, and ProjectRed are three other very logic-heavy Minecraft mods

Minecraft Pi Edition was an ARM-only release of Minecraft Pocket Edition that is scriptable using TCP sockets, with APIs for various programming languages. Unfortunately, it was dead by the time it was finished, as it never got any updates after the initial release, doesn't have ARM64 support, and is a very limited version of the game https://www.minecraft.net/en-us/edition/pi

Minecraft Education edition has a Scratch-like programming system, but it's only available to teachers and is another thing they sorta dropped the ball on https://education.minecraft.net/en-us/resources/computer-sci...

Definitely Paper. Working with Fabric often requires you to get your hands dirty at the compiled bytecode level, which is very powerful for mod authors but really increases the floor for "actually building something that's fun to use" in a way that will probably just be very frustrating for new programmers. Forge is focused on client-side modding which is just inherently trickier and requires more awkward APIs. Paper really gets out of your way and lets you program basically whatever you want to do server-side without worrying at all about needing to install mods client-side. I would't worry at all about capabilities—minecraft is an extremely flexible program, especially with the chat commands system, and you can still create cool custom UIs and behaviors without touching client-side code at all.