His works proved to be very influential to fans, and later comic professionals as well...
...so, here are a look at a few of his best that I've covered (and a few more that should likely get a look at, sometime in the future!).
SupermanMetallo and the Parasite. Both storylines unfolded over many issues, and would be the issues that helped keep them in the forefront of Superman villains (after Luthor, Brainiac and the Phantom Zone villains) for decades to come.
Wonder Womantwelve labors to rejoin the JLA, and stuck around for a few issues after....enough to have two Wonder Women meet, and the adventures to switch to World War II (as was going on TV with Lynda Carter at the time). Marty didn't stick around for too many WWII issues, though....
Doctor Fate and EclipsoDr. Fate (and later wrote the Dr. Fate back ups in Flash in the 300s), and had the Metal Men face Eclipso for two issues of Metal Men, all with art by Walt Simonson.
Batman relatedMan-Bat and the Joker
(for all of an issue each; writing Man-Bat's later team-ups with Batman and Superman as well in Brave and the Bold and DC Comics Presents).
JLA/JSAJLA and JSA, with those teams working with the Marvel Family and the Legion of Super-Heroes, respectively, in Justice League of America.
Superman Team-Upsthe Flash, and later issues as well, including a team-up with Wonder Woman, and issues with Man-Bat, Plastic Man and Metamorpho.
Action ComicsAction Comics 500, a tour of Superman's life, and the follow up issue to that as well, showing his strength as a writer, taking what was there and finding the interesting things in it!
...including his revival of Swamp Thing in Saga of the Swamp Thing, or in his run with Blackhawk in Action Comics Weekly and Blackhawk's own title....