Dennis O'Neil in his prime in the early 1970's, in the era he was doing his best work on DETECTIVE COMICS, BATMAN, GREEN LANTERN, SUPERMAN, WONDER WOMAN, JLA, WEIRD WORLDS, THE SHADOW, JUSTICE INC., and other great work at DC.

O'Neil left DC in 1980 to work for Marvel (he was insulted that the Superman movie adaptation was never even offered to him, and left as a result as soon as his contract ended), and at Marvel he edited BIZARRE ADVENTURES, DAREDEVIL (teaching a lot about plotting and writing to Frank Miller), MASTER OF KUNG FU, MOON KNIGHT, SPIDER-WOMAN, POWER MAN/IRON FIST, and ALPHA FLIGHT, from 1980-1985.
All of which were among Marvel's best titles at the time he edited them.

O'Neil also scripted AMAZING SPIDERMAN, IRON MAN (the only O'Neil run I didn't like). And DAREDEVIL during the run from when Miller left in 1982, until Miller returned and did the DAREDEVIL: "Born Again" storyline in 226-233.

O'Neil then went back to DC and edited the Batman line of titles from 1986 until his retirement. During that time, he also wrote THE QUESTION and AZRAEL series, along with scripting a Batman story here and there.