Pokemon, at least up to gen 6 anyway. Completing the Pokedex in each gen opened my eyes to the fact that gamefreak had built in a ton of little mini games in the form of Pokemon. Catching feebas for example is a whole fucking thing in Gen 3. A game within a game

I rarely 100% anything but I had fun playing all of dredge.

Metal Gear Solid 4 was tremendous fun to replay and complete. Beating the game in various ways unlocks new gear (that's maybe a little broken) for your next playthrough.

It's a linear game, so I could see it getting repetitive for some people, but it kept me entertained.

Sidenote complain about the world: Sandbox games combined with my autistic obsessive compulsive need for completionism lead to me playing them with very frusterating habits lmao.

I feel this so hard. I really enjoyed Subnautica, but I think I spent more than half of my playtime farming a giant locker full of every single material because I thought I might need them later, but it turns out that you barely need anything to complete the game and there isn't an endgame to speak of. Whoops. It could have been justified if I were into basebuilding, I guess, but that's not my thing.

Another issue I run into is overoptimization--just figuring out the cheese strat or collecting so much that the game becomes trivial. I think that's why I enjoy Resident Evil so much, since survival horror games are designed for you to be a compulsive loot goblin and therefore don't fall apart even when you collect every last bit of ammo.

Speaking of Resident Evil: with Resident Evil: Village, I didn't get true 100% because I don't like the arcade submode The Mercenaries, but I had tons of fun doing a bunch of NG+ runs to max out all the weapons and get all the unlockable collectibles through the main campaign. I find the process of fighting through the first run with the intense experience of not knowing anything and then continuously getting more experienced and more powerful until you're sprinting through and headshotting enemies with your infinite ammo revolver on the highest difficulty by run six.

Others have mentioned SM64 and Super Mario Galaxy, and I'd also add Super Mario Wonder. The final secret level (unlocked after you complete everything else in the game) is a bit frustrating due to the sparse checkpoints, and there's one or two treasure hunt levels where I had to look up a location or two, but I appreciate that otherwise I could 100% the game just using my platform skill and natural loot goblin instincts. I did my whole run through on Ryujinx and it ran perfectly, too!

One that I wouldn't recommend for this is Control, even though I enjoyed the game and it's got a very cool atmosphere. They have this very unfortunate system where they offer four randomly generated generic missions (Kill X of Y enemy [in Z location] or get X kills with Y weapon/ability) and each mission awards a random selection from a tier and an equippable skill/weapon enhancement. You can see both the exact mission as well as the tier and category of reward before accepting a mission (of which you can have three active) and there's no penalty for dropping a mission. This led me to sitting at the assignment board endlessly regenerating missions until I would get a full loadout of missions with the highest tier rewards and reasonably achievable objectives, after which I'd go do those missions, rinse, and repeat. I would not be surprised if I spent twice as much time doing this than actually playing the game normally, and I sank a good 60+ hours into the game. Oh, the worst part is that the rewards aren't unique and there's unspecified variability within tiers (so a tier VI shotgun enhancement could be like +35% damage, but then you get a +36% damage enhancement and are like, "wait, how high does it go?"). And also, you can only see the broadest category type, so the odds are high that you'll get an enhancement that you don't even want. It's funny--I suspect the devs made the system flexible to avoid forcing people to do missions they don't want to, but it created a perfect trap for people like me.

If you liked A Link Between Worlds then Zelda: Majora’s Mask would be a good pick I’d say. Fantastic game with excellent atmosphere that still holds up today, and it is heavily weighted towards side quests and stuff in the overworld rather than dungeons like other Zelda games. The game runs on a time based system so each character has a unique schedule, and while it is not necessary completing every character’s side stories is very rewarding.

Second pick since you liked Spyro would be A Hat in Time. It’s a level based 3D platformer with adorable characters and cell shaded graphics. It’s pretty short and not overly stuffed with collectibles. I think its maybe the only game I have ever gotten 100% achievements on because it’s so cute and fun to play, while not suffering from issues say DK64 has of having just too much stuff to collect.

Stardew Valley. You could argue the game's "story end' is completing the community center but there's literally a "Perfection" post game ending

This is a tough one for me. I used to platinum games on ps4 for fun but after I moved back to pc gaming I don't really go for achievements.
I tend to enjoy beating games in general. Especially harder games or at least games that are more obtuse and such. Going back to beat jrpgs on the nes and other emulation I've had a load of fun with. I also enjoy the grind which a lot of people tend to dislike I tend to 100% if I feel it is well implemented and doesnt feel like a chore then i'll go for it. Circle of the moon I 100% in recent memory. Several playthroughs to get the 100℅ depends on the game tbh.

Risk of Rain 2 is a third-person shooter roguelike (especially in co-op) with a very fun metaprogression where you get new characters, alternate moves for those characters, items, and variant modes by getting achievements. I deliberately went out of my way to do everything, when I normally wouldn't.

Vampire Survivors just worked for me, and I enjoyed continually playing through the game after "finishing" it to unlock all the secret characters and upgrades (though I did have to look up how to do a lot of them).

God of war (PC) was the last one that I 100% and horizon zero dawn before that. Both were rewarding and didn't feel like a slog. I used to want to 100% everything also, but I've gradually moved to only doing as much completionist stuff as I can before i beat the main story. I'll delay doing main quests until I've finished everything else up to a point but if it starts feeling like a chore then I go back to the story and once that's done I'm usually over the game for a while.

Old School Runescape. it's too long to "100%" though it has insane grinds, but it's fun once you get it.

Wario Land 4, up to and including unlocking the karaoke machine in Super-Hard mode.

I'm not even much of a completionist in general and even less so as I get older, but that's the rare game I still go back to and 100% every couple years because it's just a fucking fun couple of days. Love that game.

Brütal Legend

All three Dark Souls have very easy 100% achievements that boil down to seeing everything in the game more so than completing difficult challenges. There are one or two that are, admittedly, annoying. But it's not something stupid like "Finish the game in 10 hours!" or "kill every boss while staying level 1".

There are recognitions for finishing a playthrough without dying or using a bonfire in DS2, but that's for the crazy people. And aren't achievements in the traditional sense.