I didn't start playing these games until Dark Souls 3, but I loved it so much that I started working backwards through the series and then bought a PS4 just to play Bloodborne. Then I bought a PS5 (mostly) to play the Demon's Souls remake.
I've beaten them all, but my favourite is either Bloodborne or Dark Souls 3. I think they're the most polished and refine the staples of the series to their finest state. I lean toward Dark Souls 3 because it has most of the mechanics found in all the other games, whereas Bloodborne strips out some things to make it more about dodging and parrying, with faster paced combat. But I also lean toward Bloodborne sometimes because I prefer Yarnham over Lothric, and it has some of the best monster and boss designs.
I like Elden Ring a lot too, but I rank it quite a bit lower because I really think being open world was a detriment rather than a benefit. The world feels so empty, most of the dungeons and bosses are copy/pasted, and so many locations around the world are made up of reused assets. By comparison, the other games feel very hand crafted whereas Elden Ring feels thrown together.
Sekiro is definitely the hardest. I did finally beat every boss, but then I lost my save file and I really can't bring myself to do it all again. I think I'd cry if I tried. Sword Saint Isshin took me so many tries to beat, and the Demon of Hatred was an absolute nightmare. I never want to have to fight those two again.
I'm currently on my second playthrough of Demon's Souls. I really love the remake, but there is so much jank in that game that was carried over from the original. Even though the graphics are a lot shinier, you can tell that From Software really improved on their designs with each subsequent game.
Also, frig all rolling skeletons to heck!
I definitely have more to add to this after but right now I just wanna say that Dark Souls 2 is overhated. It was my first darksouls and I love it, I actually would rank it as one of my favorite soulsborne games.
I have a friend who is very vocal about how overhated Dark Souls 2 is and that it is, in fact, the best soulsborne game (in his opinion). I don't dislike it by any means, and I even like aspects of it more than Dark Souls 1, but it's far from my favourite. Mainly, I think the art direction is way better than the first game, which I consider to be quite ugly even for the time it was released. Majula is a really beautiful hub location, and it's not the only gorgeous area in the game. The closest DS1 has to offer is Anor Londo, but I still don't think it compares.
But I think the most valid complaint about DS2 is that most of the bosses are just "dudes in armor". One of the reasons I love the soulsborne games is for the varied bosses, and DS2 really has very little of that.
My only other complaint is the hit boxes aren't quite right. I've been flattened too many times by an enemy attack that clearly landed far to the side of my character.