Justice League Unlimited (2004-06)
39 episodes, approx 22 minutes each, split over 3 seasons.
A continuation of the excellent Justice League (2001-04) animated series, set approximately two years after its end.
As you'll see in the accompanying image, while the core team remain Batman, Superman, Wonder Woman, etc, there are a lot more supporting characters than before. And that's just a sample. There's even more in the series proper, some that are obvious choices and some that certainty aren't.
The extended roster is both a good and bad thing. It's good because with new faces come new challenges, which can open the door to new types of story, that can in turn give rise to internal conflicts and additional depth.
Some examples are the stories involving The Question (Jeffrey Combs) and Huntress (Amy Acker); and the love triangle that revolves around Green Lantern John Stewart (Phil LaMarr), Vixen (Gina Torres), and Hawkgirl (Maria Canals-Barrera).