What do you mean by hardest features? It's been ages since i looked at Esperanto, but back then I didn't find it particularly hard to learn.

I maintain that the most realistically usable artificial global language would be something inspired by toki pona, but with actual effort put into picking words that are maximally understandable and pronounceable by as many people as possible and is designed to be incredibly difficult to mangle beyond intelligibility.

No even remotely complex rules, nothing fancy whatsoever, just an engineered caveman speech that literally anyone can learn from a pamphlet and reliably use to communicate basic normal things with anyone else. Like the way you end up talking when you just barely know a language and need to communicate "i'm allergic to shellfish, can you guarantee that my food won't have touched any part of an animal with a shell?"