Your answer is probably going to depend on the language and your learning style and preferences.

Personally I’ve found Duolingo to work pretty well as an English speaker learning esperanto, I can also recommend Lernu if you happen to be interested in esperanto. I’ve personally never had much luck learning other languages no matter how I try so I can’t really offer any recommendations for other languages, but it’s probably worth to keep in mind that what does or doesn’t work for others may be different for you, so don’t be afraid to try out some different programs to see what works best for you