I use a paper notebook, inspired by bullet journal and autofocus for daily/weekly goals when the list stays under 20 or so items. Recently a project started ballooning into more items than this system could handle, so I picked up todo.txt a month ago. I've been very happy with it so far. The system works with just a regular text editor and keeping all the lines in the file lexically sorted, but it's also a markup format that can be used with specific tools. I keep the project-specific list synced with a symbolic directory link from the project directory tree to Dropbox, and currently use the Simpletask app to update the list on my phone. Seems to work well for everything I need.