Solitaire.

Helltaker - I got hooked on the awesome music and fantastic silliness. (It's free btw)

Dark Souls 1, Dark Souls 2 but not Dark Souls 3. The silver knight farm is brutal and I'd never do it again.

Bioshock trilogy

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%.

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.

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

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

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

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.

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.

Most Lego games. It's like a physical pain to me, NOT to 100 % them.

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.

Little Kitty Big City, the superior cat game