I am but a humble Shrimp.

[Dragon Lord]

By opening, do they mean just the very first area? Because if they're just talking about running past the wolf at the start of Bloodborne and getting through the gate, that is probably the easiest in the Soulsbourne series. Unless they mean the entire opening area of the game plus the first boss.

It's kind of a tough call on that one though. Demon's Souls initial area is a tutorial, and only the boss at the end can really be considered tough about it. Though it's very easy to beat the boss unless you're completely new, because all you have to do is stay behind it the entire fight and it'll never hit you. The only thing tough about the opening of Demon's Souls is that if you die to the tutorial boss, you don't get another chance at it, and you lose out on some bonus goodies for beating it that can give you a bit of a start in the game. So if you die, you don't get punished by having to do the area over again, instead you get punished by losing out on a chunk of items.

Dark Souls is also a hard one to consider in this, because you have multiple ways you can go, and depending on where you go, it can be the most punishing. If you wander into the Graveyard first, and you're inexperienced with the game, you're going to get your ass handed to you, and it's going to make you really hate your life. If you start with the Master Key, you can also go down and go into the Valley of the Drakes instead, which is going to be a very bad time for a fresh player (experienced players use it as a shortcut to get to Undead Parish faster). You can also accidentally wander down into Blight Town this way as well. So the start of Dark Souls can be pretty brutal depending on which way you blindly go.

Dark Souls II is another one kind of hard to judge, because you start off in a tutorial area, and then you get to the main hub, where you have two different directions you can go. One of them being the "intended" route to go first, and the other one being a little more punishing if you choose that direction without knowing what you're doing. So you pretty much have a 50/50 chance of having a very bad time if you're a brand new player and just kind of going whichever direction you discover first.

Dark Souls III probably has the hardest opening boss fight of any of the Souls games, but overall it's still not that bad. Once you beat the boss and get to the main hub, there's only one place you can go, so there's no chance of accidentally wandering into an area that is going to completely crush you. The boss is also easy to destroy if you're competent at parrying (a theme that pops up multiple times in Dark Souls III -- Parrying is king). So even though I'd rate the boss the hardest opening boss of the series, it's still overall pretty easy.

Bloodborne has the wolf that can catch you if you're not aware of it, but other than that, there's nothing too threatening at the start of the game. Cleric Beast feels like a hard boss fight because of how HP bloated it is for an early-game boss, but it's overall very easy to fight. It's attacks are pretty easy to avoid by staying close/behind it, and if you have the uhhh, forget the name of the weapon, but it's the one that gives bonus damage to Beasts, and also use up any Molotovs you pick up along the way, and you'll get through him pretty easy. Getting through the area is pretty easy too, since Parrying is so OP in Bloodborne, and the parry window is incredibly forgiving compared to the Souls games, so even if you're not good at timing, you can parry pretty early and still get it.

Sekiro I can't comment on, as I've yet to play it.

tl;dr - None of the openings are particularly hard unless you're a super scrub or completely brand new to the game.

Also because you pointed that out, I made sure that this particular post of mine is the 1,337th.