I have started and quit Duolingo 2-3 times. Their weird and kinda useless sentences put me off. Also it takes them forever to add new courses. Although it is addictive, I don't consider it a great tool and think you can do way better with other tools and methods. For me it feels like Duolingo is for when you are too lazy to do the real study.
Memrise is only good for acquiring vocabulary. I like to make my own flashcards with the words I encountered myself and with the translations I've checked in a few dictionaries to be more sure, so I use Anki instead.
Didn't have chance to try buusuu too much, since it requires payment after first lesson.
Take a look at fluentU. I love learning new vocab and training overall language comprehension with videos with subtitles in the target language and that's exactly what this website is good for. For some languages it's really hard or nearly impossible to find videos with subtitles in the language. The downside is it doesn't support that many languages and you can't watch many videos for free
Also take a look at Learning With Texts. It's a free tool where you can add any text in any language and look up words as you read, add them to your vocab and them into Anki format.