Got to say, if we define the phrase as the ability to consistently match 5 of the most god-like beings in comicdom with challenges big enough to legitimately requiring them acting as a team to save the day, the writers of JLA from that New World Order arc on were, arguably, the most creative team or series of team writers of the past 20 years.
Every successive appearance of DC Martians as the centerpiece of the story for instance, the arc got more intense, better at allowing individual members to showcase what they could do, and more compelling.
They went from few, to many, to the one without lessening any of the "hits-you-after-a-moment" horror, or awesome, well thought-out, fan-service-y action.
Near as I can tell, they did that for nearly everyone else, too!
I'd say Starro. At full power, he was nigh invincible, able to even overwhelm the likes of Desparo.
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