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?
Dr. Fate was initially not magical at all, he was atomic! Nabu taught him ancient forgotten sciences including control over the power of the atoms. So, he was more of a cross between Captain Atom & Firestorm. Wotan was said to have the same knowledge. There are some stories where he actually discounts the existence of things like ghosts! I guess the writers were really going for the idea that a sufficiently advanced science would appear to be like magic to a primitive culture.
The thunderbolt was powerful but his powers were limited to having to being commanded and a time-limit. He did sometimes fudge the rules a bit in order to help Johnny. Whether he was actually magical opposed to due to his being from the 5th Dimension thus bound to different scientific rules and limitations is an interesting distinction I don't know if really explored.