Reportedly, dual n-back. I cannot confirm this, however. My focus improved by taking up distance running, but that's not an activity for everyone. To me, it's meditative.
There was a study that found that after intense physical exercise, concentration was better in ADHDers and worse in non-ADHDers. Regardless, regular exercise is the single best thing you can do for both your mental health and your mood.
I have heard a lot of ADHD people anecdotally benefiting from Dual N-Back and have started using it recently myself. It really feels like a workout!
https://en.wikipedia.org/wiki/N-back