I can't choose, but I gotta say the first Dark Souls makes a pretty strong argument for the 7th gen on it's own. It held a certain charm that hasn't been matched by future instalments and tbh I don't think they even attempted to replicate that. Yes, the sense of dread and despair is present in current soulsborne titles, but Dark Souls had a certain... sombreness? the world felt indifferent to your "quest", almost cold. The calm before the storm made that game what it was just as much as the iconic boss battles imo. The difference in the menu themes between 1 and 3 is the best way to explain this I think. Each is going for completely different tone straight off the bat.
< Dark Souls
< Dark Souls 3
The first entry had more a "through the looking glass" vibe, whilst future entries leaned more into epic dark fantasy imo.
Plus it had some of the best exploration and world design in the series. Jesus the first Dark Souls was a magical experience.