Elden Ring is the best one for beginners, because if you get stuck at a boss you can just leave that area and go somewhere else. You are very rarely ever “stuck”. That is not the case with any of the other Soulsborne games.
In addition, it has spirit summons which make the game significantly easier.
And since it’s the newest one AND has a DLC coming out soon, it’s also the most played one right now. So finding other players for co-op is easy.
I also consider the bosses, on the whole, to be the easiest of all of the FROM Software games.