My oldest todo app that I can think of was just a microsoft sticky note[1] on my desktop. It always stayed on my desktop. It lacked sync between other devices so that sucked but my pc was the only device I had at that time so it worked for me. After I got a phone; google keep has kept me company. It's dead simple, cross platform, and gets work done. The only fault? It's by google, and they have a long history of killing products.
[1] https://apps.microsoft.com/detail/9nblggh4qghw