I can't choose between credits scene and 100% mark because it really depends which genre you play. In games like GTA there is clear way to check you completion percentage, but in sandboxes there is no such thing. For example, I've been playing Slime Rancher lately where plot exists, but it seems just like a nice addition to a gameplay. According to https://slimeranchergame.net/ most quests are just tutorials and guidelines in such types of games. Here's another example, strategies, where finishing thousands of maps and scenarios doesn't define completion status.