This will come up in any Hackernews thread that mentions spaced repetition, so I might as well mention it to get it over with.
Supermemo [1].
SuperMemo is the king of spaced repetition software.
Anki uses the algorithm from an earlier version of SuperMemo - SM2. The current version of the algorithm used in SuperMemo is SM17.
The benchmark for the algorithm's performance is the repetition workload. I don't have the numbers close hand. But the point is that the repetition intervals in SuperMemo will increase much faster than in Anki, but you will have the same, if not better, retention rate.
Also, it has Incremental reading.
To be honest, I used Anki on and off for more than five years. SuperMemo scared me, with its ugly non-conventional interface. But in the end, I succumbed, and now for the first time, I actually do my repetitions every day for 4 months now.
This despite the fact that I have to boot into Windows from Mac OS every time I need it.
It helped me learn Python and a bunch of other less practical things.
Of course, spaced repetition is not all you need for better learning. Most of what you need to know is covered in the Coursera Learning How To Learn course.
Jim Kwik is also good.
[1]: https://supermemo.guru/wiki/Main_Page