If you mean 100% achievements on Steam for example, I really enjoyed doing that with the following games:

• Slipstream (2018): arcade racing game, 7.5h to 100%

• SpongeBob: Battle for Bikini Bottom - Rehydrated (2020): platformer with some collectathon elements, 13h to 100%

• Polyball (2017): think Super Monkey Ball, but a bit faster and stronger momentum iirc, gets quite difficult later on, and although the amount of content isn't ridiculous, it's very very hard to manage the requirements for 100%. My playtime is 59h, but I kept playing after 100% to get into the top 10 leaderboards on a bunch of levels, so I think it was more like 30-40h for 100%.

• The Stanley Parable (2013): narrative game with some unconventional puzzle elements, 40h to 100%, but not really: one of the achievements is "play the game for the entirety of a Tuesday", so that adds over 24h. Another achievement is to not play the game at all for 5 years. Some people love this silly stuff, some hate it, up to you :)

• Firewatch (2016): narrative game with exploration and some puzzles, 6h to 100%.

Any doom

Solitaire.

Bioshock trilogy

Ace Attorney games.

Why? The padding is there for those with the stomach for it, but you can just… play a new game once you’ve stopped having fun?

Sekiro

I burned myself out of many a game as a kid attempting (and usually succeeding at) 100% them, so I learned my lesson. Nowadays I just play for fun and maybe go after the platinum if I liked the game enough. My time has value and I'm not going to squander it to "look at the minimap, go to waypoint/marked location, collect random collectible, rinse and repeat for 10-15 hours", nor am I spending dozens of hours grinding some random activity.

For example, I did almost 100% all Yakuza Kiwami. Did all side quests and enjoyed most of the random activities! Iirc those I had most trouble with were karaoke and billiards, but I still had tons of fun learning them and gradually getting better at them. But fuck me I'm NOT going to grind the coliseum for hours just to buy random weapons that I don't need but are arbitrarily required for the 100% completion.

I've also learned not to rush it. I frequently replay games that I like, so if I miss an achievement, that's fine. Maybe in five years I'll pick the game up again and grab the random achievement I missed the first time around. There's no need to sweat it, no need to read guides before/while playing the game and potentially spoiling me some major story events, and no need to immediately replay the game just to reach that random achievement.

That being said, the game I had the most fun with was CrossCode. Movement is fluid, combat is snappy, story and characters are fun and puzzles are actually challenging. It's the most charming experience I've ever had playing a game, and it's why it immediately jumped straight into my top favourite games ever. There is technically a "completionist list" within the game with some abdurd and missable requests, but it's not required to 100% the game and it's basically just a pile of challenges that you can tackle on if you feel like it. If you don't, you can just play the story, do the side quests and collect all the treasures, which is a lot of fun. The platinum only requires you to beat the story iirc, so you don't even need to do any of that; I did it because I liked the game and wanted to do it. Twice! I bought the game on PC and console and 100% it both times.

Most other games that I enjoy completing are platformers/collect-a-thons, such as Spyro, Crash (except Crash 1, never managed to finish it, but had tons of fun nevertheless), Ori, Celeste, PS1 Oddworld games, etc... and metroidvanias, if they are not overly long (basically every one I've played except for Hollow Knight).

The longest run I had was Final Fantasy XIII. I liked the game so much that I kept going back to it over the years, slowly chipping at the side content and grinding my way to max level. I got the platinum almost ten years after starting my save file, and I wouldn't have enjoyed it nearly as much if I attempted to rush it all in one sitting.

Sable was a perfect open world that I 100% in about 30 hrs. Totally didn't overstay it's welcome. I think they added a fishing quest after I played it so not sure how much that changed things.

The only game I've ever done 100% Mario 64

Escape velocity (1996, ambrosia software game for macOS). My favorite computer game of all time I think.

I don't think I've ever 100% a game. I don't care about achievements and I've finished the campaign/story on many but I rarely go back to any.

Armored Core 6 is pretty easy to 100% and it's really enjoyable the whole way there. Just don't bother with PvP.

And my favorite 100% grinding game that I keep playing just because I enjoy it so much, is the Earth Defense Force series. The game encourages you to play each mission with each of the 4 classes in each of the 5 difficulty levels. Even though clearing Hard also gives you Easy and Normal completion too, you're still looking at over 200 hours of gameplay, easily. The games have over 100 missions, tons of weapons to gradually improve, and at higher levels become true combat puzzles to solve with said weapons. Cheese is a way of life, in an enjoyable way.

Only games I ever 100%ed were....Assassins Creed 1 and Rocket League, strictly by achievements only. Like I never made it past Champ in RL, and AC1 was really just beat game and collect flags. I never have the urge to 100% a game cause usually it's just a grind for grinds sake and that isn't fun to me

It was tough, but I had an absolute blast 100%'ing Neon White and Hades

Factorio

Baldur's Gate 1 & 2...the exploration of the wilds while so vulnerable in the first game is something I'll never get bored of while the second game just takes it to a whole other level with your power growing so immense. They still totally hold up with the extended editions!

FF7, I replay the game every 4-5 years.

Two games I have platinumed on PS4 that I still go back and play are Subnautica and Kerbal Space Program. Not sure if you mean 100% as in like scan and collect every item, or get all the trophies, cause I don't think I've ever 100% a game in that way.

City Skylines

MGS 1,2,3,4 and MGR:R

Kotor.