I played Demon's Souls first.
I've put in way more time into Dark Souls. On a strictly mechanical level, I have a stronger connection to DaS than to DeS. The refinements and additions to the combat make it more enjoyable, the openness and interconnectedness of the world plus the ways the game accounts for tackling objectives in a different order makes it more replayable, the bonfire and estus flask are genius innovations that strike the perfect risk/reward balance that encouraged exploration and experimentation, and the more robust multiplayer make for more interesting encounters.
I do regret having only played DeS many years later after its launch, so I never got to fully engage with its multiplayer aspects. I never did any co-op nor did I ever get invaded on my first playthrough since I had no idea how any of that worked, and the whole World and Character Tendencies didn't factor in at all for that first run.
Beyond the gameplay aspects, Dark Souls is also just more captivating of a world. It's got minimalist storytelling that requires an archaeological process to understand all its narratives. It has an assortment of memorable characters that weave in and out of your adventure, and each one's own journey reinforces the themes of the game. Traversing Lordran truly feels like going on an epic quest.
And yet... that very first time I played through Demon's Souls has stuck with me longer than any of the many complete runs I've had in Dark Souls.
1-1 was such an eye-opening experience.
I remember getting completely owned on the very first path up to the gate with the dregs ambushing you. I remember walking into that hole with the bright sprites on the left. I remember slowly inching through the pitch-black barracks. I remember getting firebombed to death making my way up the dark wooden platforms to the ramparts. I remember spending more than an hour trying to kill that first red-eyed knight only to find out afterwards that the door it's protecting was locked, so I spent all that time for no tangible reward. I remember the relief of opening up that very first shortcut. I remember getting blown up in that little room with the explosive barrels and the framerate tanking, making it even more dramatic. I remember getting flattened by that boulder trap. I remember getting burned alive by one of the dragons when I tried to get the items near their resting place. I remember being intimated after a giant spear came flying through the air and landed right in front of the camera, then seeing what the first boss was, going "what the fuck is that thing", and eventually realizing I had to use fire to kill Phalanx.
It had been forever since I felt that kind of struggle and despair and fear and joy and mastery in a game.
And the game kept putting me through one fresh hell after another.
There's the gauntlet that is 1-2 which I finished by shooting a bajillion arrows at the Tower Knight's head because I didn't know any better.
There's the seemingly never-ending mines of 2-1, culminating in the spider boss that drained my slow af, fat-rolling, fluted-armored ass of almost all my healing items because I kept getting caught in its webs and barbecued.
Then came the labyrinthine 2-2 which I thought I would never find my way out of, culminating in the Flamelurker that also barbecued my slow af, fat-rolling, fluted-armored ass for an hour.
There's the entirety of the Tower of Latria that made me genuinely afraid. It's too bad I only got the AI opponent for the Old Monk boss fight, but I guess it was a consolation of sorts after I hit a wall with the Maneaters, which I had to come back to later on with a fire sword and 10 levels higher to defeat.
There's the entirety of the Shrine of Storms that made me genuinely feel like it was impossible to beat because of all the flying manta rays chucking heat-seaking spears at me while these skeleton warriors shot arrows and smashed me with their giant blades on narrow footpaths on a cliffside, the ghosts that fired unblockable lasers while I was crossing narrow staircases with oblivion to each side, and the Old Hero that somehow knew exactly where I was everytime I hauled my loud clanging armor-clad ass from one spot to the next. I also shot another set of bajillion arrows to bring down the Storm King because I was just traumatized by its babies that I just sat behind cover plinking away at its majestic ass from afar.
And then there's the Valley of Defilement. Good fucking lord. Just... jesus christ. It was one long, painful nightmare that ended with one of the most unique boss fights I've ever experienced. The whole setup was chilling.
1-3 and 1-4 felt like a return to normalcy, even with their own set of challenges. False King Allant felt like a proper end-game boss, and the stripped down version of the intro music felt appropriately majestic. And ho boy was I not expecting him to drain whole levels from me, as if the game hadn't already proved how much of a motherfucker it was before.
Finally, the Maiden in Black dragged me into the abyss in the core of the Nexus to confront the Old One. I was amping myself up for one last big fight. Destroying the pathetic pile of goo that was the real King Allant then realizing I had beaten the game was a huge anticlimax. It was only until later that I learned to appreciate what the game was going for to end that way, that making the true final boss a cakewalk after all the trials and tribulations was actually consistent with how the game was an exercise of subverting expectations.
Demon's Souls really is a special game. Only Bloodborne has been able to replicate the genuine anxiety I felt my first time playing DeS.
If you were to ask me right now which game I enjoy more and which one I'd pick up and play, it's probably Dark Souls. But yeah, Demon's Souls holds a special place in my heart.