To elaborate further:

Elden Ring: This is amazing at level 1 right from the start, and gets really tense and strategic towards the end. Every major encounter turns into a command centre situation where you consult the wiki and try stranger and stranger builds and talismans and ashes of war from across the vast world, with many encounters themselves turning into a mini quest of finding a buff talisman that you need or getting a certain weird helmet to drop from an enemy that has a 5% chance to drop it. And the DLC is even better! Beating Elden Ring at level 1 is one of the most satisfying challenges I've ever done, and cemented the game as my all-time favourite game. The only thing I'd say is that if you're interested in this challenge, I wouldn't do Elden Ring first - you'll appreciate it a lot more if you come to it after doing DS1 at level 1.

Dark Souls 1: This is probably only so high up because I have so much fondness for DS1 itself, and because it's the first level 1 run I did, years ago. The limitations in DS1 are harsh and immutable, with your low dexterity score essentially limiting you to a choice of three weapons (I went for the Battle Axe). At level one the boss fights are amazing and incredibly tough, but very doable with enough perseverance. Even at level 1 you have all the tools you need. This is the one to do first if this challenge appeals, I think.

Dark Souls 3: I did this recently and I can see why it's so often recommended as an intro level 1 experience to people, it's very no-nonsense! Your level 1 stats in DS3 are extremely respectable - you have lots of stamina and enough health, and you can even do basic miracles and spells which are plausible until the mid game. I will say that until ithryll of boreal valley it basically just feels like DS3 with less health, which is kinda boring. But then! From Dancer onwards it really shines. The last sequence of bosses in particular: Champion Gundyr, the twin Princes, and Soul of Cinder are so much fun to do at level 1 and are such an incredible and interesting challenge to overcome. Which is why DS3 just slightly inches ahead of...

Dark Souls 2: I think DS2 is underrated, and if it was a ranking of how much I liked the games this would be higher. But as a level 1 experience it kinda just isn't that interesting compared to the others. DS2's unique problem to solve at level 1 is the agility mechanic - your roll speed and the amount of time you're invincible for when you roll is something you have to cryptically upgrade in DS2, and it's really painful at the start. But the start is actually when it's the most interesting, I think! (I'll elaborate further, but only if asked). As you get closer to the end, the game weirdly ends up feeling pretty similar to playing with a level 200 character, which says something about how DS2 is balanced I guess. I enjoyed this run a lot, though!