It's main strength when it comes to teaching is that the language itself is limited to the basic constructs, so there is no confusion around really similar ones like there is in many other languages. So there is a lower learning curve for getting something productive into code.
From there, they can go just about anywhere since Raspberry Pi supports all of the major languages, and a good lot of them are already installed by default since they are used by the OS to begin with.
The other thing is that they could also do some electronic projects if that ends up being of interest. The Pi is pretty awesome for that.
I don't know if best, but I know it well and with the repl and existence of notebooks like Jupiter (https://realpython.com/jupyter-notebook-introduction/) it gives me the tools to get started and teach quickly.
Interested to see what resources others share as I not using any right now just my own knowledge of python. I did have kids learn scratch using a book as a prequel.
I did come across this book but still have not decided if I will use it:
It's essentially a puzzle game but you beat it by writing some code. It uses LUA a popular language for gamers (is used primarily in Roblox).
[1]Captain Code: Unleash Your Coding Superpower with Python:
Grasshopper is nice because it teaches concepts well, is basically JavaScript syntax, and is largely button pushing through little puzzles, typically visual / animation based, which can be more motivational for beginners because they can see their code working
Would be interesting to see if it's approachable for younger humans