10. The Lover's Quest - The Legend Of Zelda: Majora's Mask
Side quests have been a thing in video games as long as there have been games longer than three hours. They're the little stories within the larger narrative that grant you rewards the main story doesn't, flesh out the world a little by showing what people NOT associated with the quest to slay the Ancient Dark Lord Ticklefloof are up to, and establish secondary characters you otherwise would not have come across.
But the main story beckons all the same, so ideally you shouldn't want to make the side content more interesting or more engaging than the actual main game itself.
These ten games should definitely have remembered that part.
A main story can feel like you're on a railroad at times, so it's usually the player's choice to engage with content beyond the primary serving. Or maybe the main story is just kinda bad and the side quest where you sit and watch paint dry is a million times better in comparison.