I'm using Windows 10's Microsoft To-Do...I don't like the fact that there's no copy-paste feature, but the simplicity is nice (at least there are reminders and an easy way to check it off). I wonder if anyone else here uses it. I'm pretty much looking for a better app myself.
When it comes to coding to do's, though, it's often so much that I simply use Sublime Text (it's convenient - you don't need to save it even if you turn off the pc, it's offline, and it's right in my face anytime I do anything on a coding project. To tick a to-do off, just use // or /**/
;)