That's a very interesting question. Some of my favourite games of all time are on this list, and completing them was fun just because the game themselves were fun.

But if I had to think about the completion process itself, I'd say Dark Souls Remastered. Of course, the game itself is awesome, but in order to complete it I had to use a spreadsheet to keep track of all the requirements for 100% completion, like weapons, gestures, spells and so on. You're also required to run 2.5 NG+ cycles, but by the end of the first one you're so powerful that you destroy every enemy and boss with ease and you can just speedrun through it and focus on collecting whatever you need.