What is the alternative to Duolingo that has worked best for you? I’m especially interested in learning Japanese

I agree. This website has been very useful and encouraging on my language learning journey.

I feel like it depends a bit on the language you're learning, which one you're coming from, how your brain works, and how much you're willing to think about the exercises instead of just doing them.

I'm currently using Duolingo to learn esperanto, which is admittedly a bit of an oddball language since it's a constructed language and the grammar follows pretty simple rules. Ive been at it for a bit over a year and while I still have a long way to go, I feel like I'm well on my way to being fluent, mostly I need to pad out my vocabulary more, and my ear could use more training.

It helps that a lot of the words look/sound pretty familiar to me as an English speaker, and the kind of person who likes picking the sentences apart a bit and actually figuring out the grammar and such as I'm going.

I took a few years of french in high school, none of it really stuck, and I think the traditional classroom approach just doesn't work very well for me in learning a language. But I think if I had something like Duolingo at the time where I was just kind of thrown to the wolves and made to figure it out myself I would have been more successful