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

If you are just getting started, this is a good resource for learning hiragana and katakana.

Past that, I used Anki and Bunpro for learning vocab and grammar. However, an alternative to anki for vocab that's definitely worth checking out is jpdb.io, and Cure Dolly's youtube videos are good for learning grammar.

There are also some decks that people have on anki which have sentences that you can practice on, I hear those are a pretty good way to start reading so that you can work your way to reading books/manga and stuff.

Here's another website that's worth reading through if you're interested in doing immersion learning with japanese.