Which horror series has.....
- deviated most from it's original concept.......the original horror movie....
- has stayed the most true to it's original concept......the original movie.
I think the first one has to be Halloween. They took a story about a guy who escaped a mental hospital, returned to his hometown and killed babysitters into a whole mythology about some unstopable evil needing to kill his family members. A very far cry from the original concept.....
I'd have to say the second is probably Hellraiser. With 6 movies released........all follow the same concept. Evil Cenobites from Hell led by Pinhead come to Earth to claim those people whose greedily lust for all pleasures leads them to open the puzzle box.