What is the best mobile free app to learn english ?

Hi All

I'm not an English mother tongue so I have to learn the most used worldwide language.
My level is medium and I am searching continuously a good free mobile app to learn it.

Actually I am using Duolingo  a good application but very basic.

Recently I discover a new Application named Knudge.me. It is not totally free but in free version permits a good learning level.
Application presents some games to learn new words, it permits read a little text and answer a some related questions.
I like it. I am using it for 3 days but I think it's a good app.

In the next day I will have an idea more specify while the question remain "What is the best mobile free app to learn english ?", do you know the answer ?

Stay Tuned.