I don't know.
In many cases, I find the obscure villains I use through research, or because my memory dredges them up to fit a particular story I have to tell. In some cases, I'll like a villain not because they've had any wonderful stories, but because I see something I can do with them that makes them more interesting. [For instance, I can't say I've ever much liked the Prankster, but I recently came up with a way to use him that makes me like him.]
And, of course, I'm plenty open to using characters from other places -- I used Dr. Cyber without any connection to Wonder Woman, and Kulan Gath as an Avengers villain even though he hadn't been one previously.
So I dunno. I'd pick Amalak, I think, on the grounds that I think I could do something with him, and Microwave Man just because I like him. But most of Superman's decent villains seem anything but underused.
Let me turn it around, though -- who are _your_ favorite underused Superman villains, and why? Question open to anyone...
How about Mohammed Ali?
Was that tabloid comic great, or what? Man, they ought to turn the Superman/Ali fight into a regular event, sort of like the JLA/JSA team-ups (hint, hint, DC).
I always loved King Krypton, who, after Mort Weisenger left the super-titles, was apparently forgotten forever.
He was a scientist from Krypton who had his brain placed into a gorilla, so when he came to earth he was a superpowered, talking gorilla. WOW.
I always loved the Galactic Golem, too, who I think was created by Cary Bates, and who was the only person that tended to use him. I found it fascinating that he was made of something that could be transmitted by radio telescope. He had a wonderful look to him as well, with all those sparkling stars and miniature planets.
Diana Savage is one of my favorite of the "Jimmy Olsen, Mr. Action" villains, especially when the art is done by the wonderful Kurt Schaffenberger. Jimmy, I think, is at his best when he's a two fisted tough guy instead of as an idiot who drinks potions on alien spacecraft, who makes you wonder if Superman's constant rescues of Jimmy from peril is interference with natural selection. Can a tough guy wear a bow tie? For Jimmy's sake, I hope so!
I did also love that civilization of long-lost Vikings in Canada that rides giant birds, created by (I think) Elliot S! Maggin.
My favorite Legion of Super-Heroes enemy has to be Otto Orion, who had a heck of a better story than the unpromising "world's most dangerous game" concept would lend itself to. Natives with ray guns built into spears? That splash page with the Legionnaires' heads are framed? The splash page against a horned tiger? Panel on panel of monsters, from radioactive dinosaurs to eagles trained to drop kryptonite cages? Wow, this was all great stuff; nobody could do creatures like Curt Swan could do creatures. Little Jimmy Shooter was as much a genius as he is adorable.
I'm not a very big Gerry Conway fan at all, but I did find absolutely wonderful that one story arc in his JLA (making him a Superman enemy by proxy) where he had half-human, half-animal monsters. They had a half-rhino that gored the Flash, a fight by Hawkman against scorpions with human faces (!) and Superman getting punched all the way to the great lakes by a Godzilla-sized whale monster. While a catgirl is not the most original idea in the world, the personality Conway gave her WAS; she was skittish, and craved body heat, and warmed up to Superman immediately like cats with people. And of course, the leader is a lion-man named "Rex." There's something wonderfully cheesy about that; it smiles at you and you can't help but smile back.
But you must admit, he was utterly overshadowed by Thunderboot!
Where does the Mad Thinker place the Classified Ad to GET these people? "Must wear hammer on head."
Very glad you liked her!
It's true! Silverclaw is one of my favorite characters, along with the Valkyrie in Englehart's DEFENDERS, Captain Comet, the Japanese Dr. Light, and (obviously) Superman. Does she actually TURN to silver, a la Ferro Lad, or does she just acquire a peculiar silver color, or what's the story? Can she turn into an electric eel and get electric powers (that's a South American animal)? Or into a chameleon and get color-change powers (ditto)?
Speaking of wonderful obscure characters, whatever happened to the Martian Manhunter's Silver Age sidekick, that little orange alien Zoot, with the trumpets on his head that can create cold? To the best of my knowledge, he has not been brought back or even mentioned since the Martian Manhunter's spy series in HOUSE OF SECRETS.