Tetris apparently

Final Fantasy VII. I was never able to beat the sephiroth boss fight. I enjoyed the story, but I don't feel the need to play it again.

I was waiting to finish totk until the DLCs came out.

I'm still waiting.

The full grand campaign in War in the Pacific: Admiral's Edition.

Ypu fight the entire Pacific War in 3 day turns across a map with 40 nautical mile hexes with the full order of battle at your disposal. If you play seriously to win against a capable opponent, you have to manage everything right down to individual pilots. There are thousands of ships, thousands of planes and pilots, and hundreds of land units. It is almost more of a historical achievement than a "game". You can play against the AI, but the real experience is to play against another person by email. The full campaign could take 450 turns if you play it out right to the end. Of course, one side can concede early if defeat becomes inevitable, but even so, if each player takes one turn per day, it will probably take about a year to finish. And you won't just be spending time taking your turn, you'll also be reading up on the actual war strategies of each nation, analyzing your opponents moves, planning your own moves and strategies, figuring out supply logistics, learning the capabilities of the ships and planes at your disposal, reading intel reports, and managing virtual pilots and commanders. In many ways, WITP:AE is more of a technical and historical achievement than a game. I honestly don't know if I'll ever be able to complete the full campaign. I would certainly have to be retired before I'd ever attempt it versus a human opponent. And even if I could fully commit, who knows if my opponent would also have the fortitude to finish.

The Celeste mod Strawberry Jam. It's a lot of fun, but I'd be more than satisfied with just getting through the expert levels, which are already at the point where simply watching gameplay of them would kill a small Victorian child, but I'm at just the right level of masochism that this is a tough but satisfying grind. But I've seen what's next, I'm fully aware of the horrors of the Grandmaster levels, and I know I'll hit my upper limit well before 100% completion.

I honestly have no idea if I ever finished Skyrim. I finished a ton of quests and became an unbeatable bow hunting dragon killer who yelled all the time. I'm pretty sure I got to some castle where I had to choose a faction and just didn't bother. I explored for a couple hundred hours and never played again. 7 years later I played Morrowblivion and then replayed Oblivion (I couldn't get Skyblivion to work if I remember right). I've debated playing VR Skyrim, but I don't want to repay for the game.

At this point, FNV. Got it a year or 2 ago on PC with all DLC and I'm just now getting to New Vegas after an extended break of not touching the game.

Factorio. I'm almost at the stage where you can build the launchpad for your rocket, which is technically the end of the game. But the Pyanomod and Space Exploration mods literally add like 1000 hours or more to the game, and I just think I'll probably be dead or society will collapse before I finish those up. The game is literally infinite if you want it to be.

Oxygen Not Included

I still haven't finished Bastion because I didn't want to break the loop.

I still haven't finished Planescape: Torment despite playing it over many years because I had too much fun reloading and exploring parallel branches.

Super Meat Boy, I think lol

It‘s not my favorite game ever, but it certainly is one of the best games in my library I‘ve too much of a skill issue to completely beat

I've enjoyed a lot of Earth Defense Force, but it seems like there's a TON of missions in it. I'm up to 30 or so and have already had some big "This is our epic final battle against the Giant Insects!" moments - and it goes up to 90-something missions, plus DLC.

Undertale. Got two of the endings and got to Final Boss of Murder Mode. Tried 50+ times and decided I’m just not skilled enough to beat that boss. Looked up a vid of the ending and was satisfied. What a wonderful game.

Alley cat

And

Zack McKracken

Escape From Tarkov.

I love the game mechanics. But I suck so bad, I'll never get the last quests.

Well no-one else has said it yet but pokemon.
Over my childhood i played a hell of a lot of Red, Silver, Sapphire and Leaf Green, but i never got close to completing the pokedex.

On my first pokemon (silver) i had three pokemon at lvl100, and the rest of the party was lvl95-98 playing the elite 4 over and over and over with a single pokemon just to get the last few levels (after running out of rare candies) but i never got the dex closer than ~30 pokemon off. Even disregarding the trade or alternate version exclusives and the [practically]unobtainables (legendaries from event codes) it was such a grind to get some of the last pokemon with high evolution lvls and i just never managed to finish that pokedex.

I have come to terms that i will never infact 'catch em all'.

Kingdom Come Deliverance probably

I used to like playing hard games but I’m done with them now. I don’t have the skill to complete them without putting a lot of time in and I don’t have the time anymore.

Returnal was fantastic and Sifu was even more fun but I’ll never finish either of them.

Probably Hollow Knight at this rate. That 112% is a lot.

Risk of Rain 2 for me. I’ve “finished” the final boss but there’s so much more to unlock and do in that game that I’m not sure I’ll ever be done with it. Absolute blast to play with friends especially when you get those miracle runs where you get all the items that are amazing for your respective classes!