I have been trying To Do list apps for awhile and none of them worked well for me. ToDoist seems to be the one, it's on everything and is very lightweight. It's very easy to quickly add a task, and maybe 30 more seconds if you want to put it into a more granular organization. It's free version is great with the $29 annual payment unlocking more options. Things like gps based reminders, adding notes to items and tags to organize tasks across projects.
If you do check it out you will be much better served if you take the time to watch a few of the tutorial videos.