Definitely not obscure since he was used in the movie and Secret Origin, but people overlook that during the Silver Age, Hammond was second only to Sinestro (whom he pre-dates) as Hal's greatest foe. He also acted far more like an "archenemy", scheming from behind the scenes, organizing other villains and attacking Hal without Hal ever knowing it. Meanwhile, Sinestro was trying to take over Earth 2 by possessing a yellow cab.
Obviously, Sinestro has become a much more complex character since, and Hector doesn't really make for memorable covers or fight scenes (obviously), just creepy, ugly ones. Still, I think he deserves a comeback. In his first appearance, before he became a paralyzed big-brain, he was basically an evil, fraudulent Tony Stark and almost successfully seduced Carol. Since so many modern GL stories revolve around space and the corps, it would be cool to have him back in that persona. Have him become normal, but still with psychic powers, then become a businessman under a new identity. Let Sinestro be Hal's greatest nemesis when it comes to the corps, and let Hammond be Hal's greatest nemesis on Earth.