Well, iirc, Pym was chosen as Scientist Supreme because of his unique approach on science, not just pure intellect. I'm not sure who here would meet the same criteria as Pym being "the Mage", but the argument could be made for most.
That sort of fevered, almost unstable, need to create, to build, to achieve wonders - the criteria by which he was chosen for his role as scientist supreme - most accurately characterizes Will Magnus. They even both have history of mental instability.
Steel represents the Stark-like desire to enact social change through the power of innovation. He's about transforming the global community through his creations, more than he is about the love of creation itself.
Ray Palmer is a professor and an adventurer, and really values the understanding of a concept or principal more than he values the creation of a device or object. His technology, his innovations, tend to serve as tools for further information gathering, exploration, learning.
In terms of who has created the most impressive stuff? T.O. Morrow probably takes the cake, but Atom and Niles Caulder give him a run.
I note that most DC scientists are more specialized. Atom's great but you'll almost never see him build a robot, for example, and likewise Will Magnus will rarely do something in particle physics. Mr. Terrific and Batman are generalists, but tend not to do as crazy stuff.
Hm, Professor Ivo should be on the list if we're including villains, though
X Supreme does seem to mostly be limited to heroic types.
I don't even know who Professor Haley is. Scoop?
T.O. Morrow is probably my bet of the list. Magnus does point out how awesome he is for making a robot soul, and he made a device to view 100 years into the future (though admittedly, this does mean he had a knowledge advantage in building his stuff after that) as well as a device to create space that he used on Black Adam during 52. The Red Tornado (Volcano, Torpedo, Inferno) series is quite impressive, and Tomorrow Woman is crazy-awesome (though a collaboration).
Less specialized than you might think. Besides building Time Machines, Ray Palmer's greatest feat is probably overcoming the Hourman virus, which was 853rd Cen technology. And Caulder has done everything from creating Nanomachines and robots to time machines.
He's the Professor of the Challengers of the Unknown. He's never really accomplished anything on the level of the others on the list. I believe his greatest feat was not being driven made from the information on how to traverse Hypertime, which was given to him by Metron.
Note that he first invented tesserect technology when he was 15, and altered an iPod to become a death ray. That said, Magnus makes it clear that he himself is the superior 'soul creator' when he's off his meds, and both seem to value that particular feat above all. In some ways, it makes sense, as building a synthetic soul is pretty insane.
I believe the implication with Tomorrow Woman was that Ivo created all her physical abilities, being that his specialty is creating super powerful robots, while T.O. Morrow created her brain.
His Post Crisis history of genius has been spotty at best. Only recently is he getting anything approaching a push in that regard.
If you take into account all his Pre Crisis stuff, and htose Pre Crisis stories appear to be back in continuity, then he would be insane, but kind of stupidly insane -- building time machines out of tin cans and assorted garbage.
Either way, none of these guys would be the Scientist Supreme of the DCU, since that is a decidely heroic/protector role.
But prior to that, there wasn't a lot. Basically from Man of Steel until the Johns/Busiek era he became a business man, a Kingpin and a political manipulator. He had pockets of brilliance - Alan Moore's Swamp Thing had him design a device in under 60 seconds to destroy the Swamp Thing during his Plant-Elemental phase (when nothing else could) - but for the most part he hasn't been that impressive.
But, again, given that a lot of that Pre-Crisis stuff seems to be in continuity again, yeah, he's probably got the most outlandish stuff of the present day DC characters.