Justice League found it's creative footing in season two, and really took off when it turned into Unlimited during the whole "Cadmus" arc.
And The Simpsons first attained true brilliance in season three. While season two has some classic episodes as well, there was still a lot of fine-tuning going on with character voices and personalities (I watched "Three Men & A Comic Book" recently, and Hank Azaria's first crack at the Comic Book Guy's voice is distractingly "off").
And Jackie Chan Adventures was improved when Tohru[sp?] became a good guy in the second season.