1. Dark Souls. Best balance of difficulty, weapons choices, online and level design.
2. Bloodborne. Best atmosphere. Most interesting story. Great combat. Lack of gear variety hurts. Bad online.
3, Demon's Souls. Still plays great. Lacks the polish of later games.
4. Dark Souls III. Really tried hard to capture Dark Souls' magic but failed. Pretty good game.
5. Dark Souls II. Trash outside of the DLC's.