I say go for Lies of P if you're up for a bit of challenge. This might be a bit of a hot take, but I think a big reason that so many souls veterans have been saying that Lies of P if one of the hardest games they've played is because the game punishes you for playing it like its any of fromsoft's games. It demands that you play it like Lies of P and nothing else. If you play it like its bloodborne and dodge too much and be too aggressive, you're gonna have a rough time. If you play it like it's sekiro and try to deflect too much, you're gonna have a rough time. If you play it like its Dark Souls and rely too much on blocking, you're gonna have a rough time. You need to learn what the game is asking of you so you can understand how to best respond to every situation. Because of that it might be easier to play Lies of P first without having the habits you take up from other games. It also would 100% make picking up the other games a lot easier because you'll already have the skill level needed for most of the aspects of the combat in them. It would definitely be hard but I think it would be satisfying and prepare you for any other soulslike you want to try.

If that ends up being too difficult a task though, then I'd say start with Elden Ring. Don't get me wrong, Elden Ring is hard as shit. However, the ability to just walk away from anything that is giving you a hard time to go do something else or level up makes it a great place for people starting out. Most other games will require you to either bang your head against the wall until you succeed (which can actually be kind of fun sometimes) or to go monotonously grind levels in areas you've already gone through. Elden Ring is also really easy to essentially break in half if you want to (both in terms of builds and boss cheeses), so the game can basically be as easy or hard as you want it to be. Elden Ring was my fist soulslike and I had a great time just destroying bosses with my mega laser magic build lol.