There are zillions of to-do lists but I can't find one that does what I want. Rather than have a limited set of priority levels (High/Med/Low, ABC, etc.), it should have unlimited levels. You should be able to add an item at any level easily (e.g., Ins 6), easily reorder the list (e.g., select item, then M(ove)6 or A(djust)6 to move it to #6 in the list, and inserting or moving an item should automatically adjust other items' priority (inserting a #3 moves the current #3 to #4, #4 to 5, etc.
WordPerfect's old DOS Calendar used to do this, but I don't know of a current program. Other bells & whistles like categories and due dates would be nice of course but aren't necessary for me.