Season 1 of Justice League helped kicked things off. However, it's the 2nd season where things really shine. Season 2 went on and improved the show's animation, added on excellent subplots that carry on such as a growing relationship between Green Lantern and Hawkgirl, and increased eye-candy such as numerous guest appearances from characters from both the Superman and Batman toons, from supporting characters like Lois Lane, Ma and Pa Kent, and Alfred Pennyworth to classic villains like Lex Luthor, Brainiac, The Parasite, The Joker, and Clayface. New villains arrive as well, such as Eclipso, Dr. Destiny, Desparo, and even Doomsday, the monster that became infamous for killing Superman in the comics.
The quality of the stories increased as well. It starts off with Twilight, as the Justice League faces Darkseid and Brainiac(Ron Perlman and Corey Burton reprising their roles from the Superman cartoon).
In the center we have highlights such as Hereafter, in which the Justice League deals with the possibility that Superman is dead. Superman, however, is trapped in an apocolyptic future, with a former enemy now his only friend, ally, and hope of getting home.
A Better World is a 2-parter that helped set up the main storyline of the 4th season. It features the Justice League battling evil, alternate dimension versions of themselves, led by a ruthless dictator version of Superman you will love to hate. However the true highlight of the episode is an intense psychological faceoff between the two Batmen.
Wild Cards features the League facing off against the Joker in Vegas in a hilarious reality TV show homage, as they try to disarm bombs he's planted while also fighting off his henchmen, the Wild Flush Gang, who in a clever touch happen to be voiced by the main cast of Teen Titans. It ends with hands down the best Batman/Joker battle ever, even putting ones seen in the comics to shame.
It all ends with the 3-part smash bang finale, Starcrossed, which end with an alien invasion(only fitting since that's what got them together) and a shattering betrayal between one of the League's members.
Since the debut of Batman: The Animated Series in 1992, the WB's series of superhero shows that have since been known as the DCAU have continued to prove that animation can be intelligent, thought-provoking, and enjoyable for kids AND adults. Justice League season 2(which is released on the same day is Superman vol. 3!) is no different. A definite must buy.