Demon's Souls is great, and anyone who loves Souls games need to play it. If you are able to play it (it's PS3 exclusive), do it. I think it's great as the first game, because DeS is like an overture, a summary or introduction of ideas that will be later re-used in later games and expanded upon. This is great, but others might feel like it's lazy or that From is copying themselves. I simply think that Miyazaki went crazy on this first game (which he did) and threw a bunch of ideas together to see what worked and what sticked (a lot of it did IMO; it also was critically acclaimed, which means a lot of people did too).
That said, I think my favorite is Bloodborne. Such an excellent game. I think the only drawback is that it favors aggressive gameplay (instead of the turtle style that other Souls game allow) and it lacks deep builds (some stats like magic take a step back). But the game is rich in lore and the gameplay is pretty gratifying.
Also, I think Dark Souls is a perfect introduction too to these games. So I think it's hard to recommend a starting point, other than accessibility (Bloodborne being on Sale helps).
Lastly, no matter what game you want to play first, make sure to include the DLC in your session. I think FromSoftware makes excellent DLC that adds so much to the game in multiple aspects: they are sort of standalone, but they are so well-crafted and so properly implemented that they never feel tacked on or that it was supposed to be part of the main game. Hard to explain without diving deeper into spoiler territory, but let's say some of my favorite moments of Bloodborne were from the DLC.
If I have to Rank the games:
DeS: 9.5
DaS: 9
BB: 10
DaS2: 8
I still need to platinum Dark Souls 2 and I bought Dark Souls 3 and Sekiro, but haven't play them yet. Waiting for a proper time to start them, as I like to devote all my time to them once I start and do and see everything in each, including Geeking about each game's secrets and lore theories and everything that makes these games rock.