Todoist. It’s nothing to do with “productivity” per se, but that is a benefit I’ve reaped.
Instead of having to remember what feels like 100 things all the time and doing 2 of them, I just remember the app. And it’s a hell of a lot less stressful. The sub-task function also helps simplify larger things and makes them simpler to tackle. It’s a game changer as an AuDHD individual.