Thought Spectre and Imp most powerful?
It's kind of hard to say since their power levels have all fluctuated throughout the years. Spectre at one point was just a ghost, wasn't he? Then he had vague magical powers. Then he was an agent of the Lords of Chaos and Order. Then an aspect of the Christian God. Green Lantern taps into the Starheart which is basically the sum of all the chaos magic in the universe, either created by or harnessed by the Guardians of the Universe depending on the telling. Dr. Fate taps into the powers of Nabu who was an ancient Egyptian sorceror but what did he tap into? The Egyptian gods, or just chaos magic? The Thunderbolt was another magical entity until they retconned him into being from the 5th dimension like Mxyzptlk. They all have cosmic level reality warping powers. Which is strongest?
A better question might be, how did the ISA beat them and why didn't the ISA steal all these power artifacts after they were dead? There was no sign of the Spectre there. Fate's helm, if it was up for grabs, has a mind of its own. The GL ring and Imp's pen should respond to anyone to grabs them, though. Or did the Thunderbolt only respond to the seventh son of a seventh son?