The second game I ever 100%ed was Arceus and I still can’t stand the thought of picking it up again, years later. The first game was Horizon Zero Dawn, which is still fun.
At least personally, its a lot of the shorter, gameplay-focused games that always leave me wanting more, or wanting to further improve, without having some unbeatable new-game++++++++ mode or anything overly RNG based.
A couple games I've 100%ed that still have significant bonus/optional content outside the main plotline include:
Inscription - Willingly played through the story twice and spent nearly as many hours on the bonus new-game+ mode. Super solid gameplay, that while well explored in the base game, leaves plenty of room to further experiment and perfect your strategy.
Just Cause 3 - while there is a ton of bonus content, its not overly hidden, and the core gameplay is solid enough that challenges feel fun and rewarding, while travelling around gathering collectables is satisfying in a chill, podcast-listening, but not unengaging way.
Hotline Miami - after completing the game, I wanted to go back and get a A+ on every level because the gameplay was fun and I felt I still had more room to grow. "The puzzle" wasn't as fun, and I did use a guide, but I was just happy for any reason to play through the game again.
Wolfenstein the New Order - again, just a solid gameplay loop that made me want to keep playing, with bonus objectives that worked as an objective rather than a chore. Also, unlike later ID shooters, it doesn't have the "beat the whole game without dying" achievement, which just feels too punishing over mistakes that may be minor or downright unfair.
I 100℅ed sekiro and another crabs treasure, both games (especially sekiro) sort of had grindy achievements, but the gameplay was so good it still ledt me wanting more
It's interesting that you mention Just Cause 3 because even though the campaign was a lot of fun for me, I didn't enjoy completing all the vehicle challenges.
The vehicle challenges were definately the weakest part, but on average, I still enjoyed them. In particular, a lot of the wingsuit courses and land races were fun, and actually took advantage of the game's strongest elements.