Surely the most innovative character since America Captain. Actually, if the shapechanging Clayface(s) eventually became Batman’s Sandman, this is Batman’s the Lizard.
After the goofy ’50s, and the even goofier (if that’s possible) Batman comics following the Adam West show, fans were ready for a more serious take on the character. They got it when artist Neal Adams exploded on the scene, one of those medium changing artists like Jim Steranko, John Byrne or Frank Miller. The writer got much more mature as well (if still very melodramatic), with Batman’s status as a detective finally coming to the fore.