I don't think Duolingo is necessarily the best, but it's definitely helpful, at least for me. I feel like some aspects of the language learning are a bit all over the place, especially with "smaller" languages such as Welsh.
I like Duolingo though because it's more fun, almost like a game. I think it's aimed a lot at children and students studying language so that teachers can encourage students to revise language in their own time, but it's also great for the rest of us too.
Other than that, the app is VERY easy to use, especially for beginners who have no previous knowledge of a language. It's also quite quick too and covers all bases.