Over the last few years I've tried just about everything, and I'd rank Seedlang second only to a private tutor and far ahead of every other app or course.
It's by the people who do the Easy German videos (which are fan-freaking-tastic - check them out on YouTube if you don't know them yet).
Oddly, I can't think of a concise way to describe Seedlang. My two favorite offerings are the "stories" and the recording function. "Stories" are little lessons, often based around an event, or a story, or funny gifs. Recording is what it sounds like - you speak the answer, then hear what you said. It's useful for accent reduction and for learning to speak without thinking. Additionally, there are flashcards, gender trainer, plurals trainer. Native speakers. Grammar explanations. Quick responses to questions. And other goodies.
Pros:
1. Excellent value for money (less than Duolingo, free version exists)
2. Really fun (and funny)
3. You can jump around levels as you wish
Cons:
1. No mobile app (but the website's mobile version is lovely)
(*Other languages are coming in 2020.)