Don't make me weigh in on this never-ending argument. Even though i'm a relic and the days of "Sir Curtis" are distant memories buried in pages 1-30 of all the Souls threads.
For me, 'the best' and 'my favourite' never match up.
I have such a personal connection to Demon's Souls (triple Platinum, different servers/playerbase each release, 5am PvP sessions in 4-1 with a beautifully esoteric crowd of Japanese players). Makes it my absolute favourite. But that's not to say its the best.
Dark Souls: The one that got the attention of the masses (myself not included having spent maybe 1600 hours on Demon's at this point) is a game of two halves. And without sounding like a broken record: everything up to and including Anor Londo (& Painted World) is 10/10! After that it falls off drastically. It's only rated so highly because thats when everyone got onboard.
Dark Souls 2: The lack of Miyazaki is evident. The 'fingered/tree branch' layout of the overworld and plethora of bonfires rubbed everybody up the wrong way. And yes, there were way too many humanoid armoured bosses. But, there's a lot to love about it. I hold it in high regard especially after the Scholar remix (which they could've easily sold as optional DLC to all the games - think about it - you know the game inside out, so why not fuck with the enemy placements to keep you on your toes? A gimmick for sure. But it keeps you playing). My absolute favourite campaign on any of these games was the 'no bonfire' run. And I challenge any of you who says “Dark Souls 2 sucks”, to play it without bonfires. Then you'll appreciate how good it is. And how good it feels to beat that boss at the end of that particular branch, just so you can warp back home.
Dark Souls 3: A nice throw back. But because of From's output, we were all getting a little bit jaded by this point. Its a lovely swan-song, but it did nothing new for the franchise. The DLC was excellent, and in my personal opinion the final boss of DLC3 is the greatest encounter i've ever witnessed, and a beautiful send off to the series.
Bloodborne: Before the DLC it was a 1-and-done game which i quickly dismissed and shelved upon completion. Only to return after they added build diversity which changed the game completely! Now I understand its utter brilliance!
Sekiro: I honestly cannot get on with it. Difficulty has never been a problem. I have never attempted a boss more than 4-5 times before succeeding. Its the regimented one note (read: rhythm action) approach which doesn't appeal to me at all. I like to experiment, and try things out for myself. Not dance to the beat of someone else's drum.
So yeah. I typed all that and still refuse to rank them. I love them all in different ways for very different reasons. Roll on Elden Ring.