Ask HN: What is the best “to do” list app/website?

What is your favorite to do list app/website/whatever and why? I am looking to improve my daily workflow and need to keep track of long- as well as short-term tasks for clients, systems etc. pp. I don´t need project management, I just need to have them in view, sync them on several devices, mark tasks as completed and assign them to specific projects (a tag "client X" would be enough)... Right now I am using a trello board to keep track of that, but that does not really work out, as I can´t easily (ok, there might be a plugin for that) distinguish between long- and short-term tasks... So... What do you use and why? I think I just need to get a better overview over the possibilities... Thanks!