Alright so i really think the first Dark Souls or Dark Souls 3 are the most "newbie" friendly and would push you to one of those. Demon's Souls, Dark Souls 2, Bloodborne, and Sekiro all have mechanics that can be off-putting or otherwise frustrating to deal with particularly for a newbie.
Demon's Souls cons: diminished health pool following death with little recourse, only on PS3 with poor frame rate to boot. Have to farm for healing items exclusively. Arguably the least interesting bosses and general encounters as it is the first game in this style and very much feels like a prototype.
pros: still fun as heck, great level designs broken up into discrete zones. Some cool mechanics and ideas (world tendency)
Dark Souls cons: original version on ps3/360/pc is rough as hell technically. Easily cheesed with magics, little incentive to be human. Weak second half that has enemies and areas that feel rushed (Lost Izalith)
pros: the one that made the formula popular, recent remasters at 60fps on consoles and an actual PC version. Solid build and item variety. Overall great bosses besides a few weak links. Interconnected world is a masterpiece.
Dark Souls 2 cons: lose % max HP on death (punishes failure, disincentives experimentation and taking risks) often have to grind for healing stones (although rechargable estus flask exists), too big for it's own good -- boring areas, underwhelming bosses, rather linear paths and level design. Slow, plodding combat compared to rest of series / generally weird rhythm to everything.
Pros: great DLC, build variety, dual wielding, music. Easiest(?) (besides a few cheap or gimmicky bosses) lots of content. PS4/X1 / current PC version are an updated version of the game with 60fps console frame rate and a solid outing on PC. Maybe you'll like the rhythm of it.
Dark Souls 3 cons: "derivative" particularly of the first Dark Souls, only 30fps on consoles, maybe the shortest Souls game (not necessarily a bad thing) latter bits kinda lose steam IIRC. So-so first DLC
pros: rechargable Estus for magic and health, great aesthetic and music. Bosses, enemies, and overall level design take a step up from DS2. DLC #2 is fantastic. Tight combat is closer to Bloodborne in speed and feel. Animations have energy and the malaise of Dark Souls 2's foray into more .. realistic and weighty animations is gone.
Bloodborne cons: PS4 exclusive, 30fps cap. Grinding for health items is never fun especially when you keep dying to a boss. Lighter on build and enemy variety compared to Souls games. Co-op requires a specific resource, again not usually a problem but could be. Some areas/encounters feel like big "FUCK YOU"s from Miyazaki. Only on PS4
Pros: trick weapons, combat mechanics (gun parry, visceral attacks, gimme-back-my-blood aggression), awesome level and world design. Cool bosses and really well put together DLC.
Sekiro cons: 30FPS cap on base consoles or an uncapped frame rate on X/Pro (X also can make use of 120hz mode yaddayadda Digital Foundry). Parry system is kinda built with 60hz in mind, imo. Bosses are too chunky for their own good & game can be punishing. Grinding for spirit emblems (ammo for prosthetics) + low carry quantity of emblems distinctives experimentation and taking risks. Only one build and it is sword.
pros: feels real good when you get in a rhythm. Only one build and it is sword. great world & level design as expected. Grappling hook. More straightforward storytelling. Skill points are fun to distribute and grant cool moves (as do prosthetic upgrades) but you'll end up with most everything anyways, just a matter of when. Monkeys with swords, guns. ~Stealthy~
Largely i point to game speed, grind and mechanics of failure for why Dark Souls & 3 are the best starting points. Demon's, DS2, Bloodborne, Sekiro all have aspects of grinding for items that get in the way of playing the game / moving further in the game. Whether that be for health items or utility, it can bring these games to a halt if you've hit a particularly tough challenge and depleted your item stores. Demon's is a bit fast while Bloodborne & Sekiro are faster than the prior game in the list. DS2 is slower than the others and is an acquired taste, but it might also be a good starting point because of its speed. DS2 & Demon's both reduce max health upon death, which can lead to a frustrating cycle of dying more cause you have less health to work with. Sekiro is all like "ahh Dragonrot" but that ends up not really mattering so it's a moot point. Bloodborne doesn't do anything like that, but the grind for blood vials and bullets is enough of a punishment to fill the same role upon hitting a tough challenge that continues to best you.