John Stewart
Best:
Pretty much all of the first two seasons of Justice League animated. Nearly everything he did and was involved with was gold. It's easily his best interpretation by a long shot, and I would say it's even easily the best interpretation of Green Lantern by a country mile, especially in media outside of comics, but I know there will be many who disagree with me there. But I've never seen a character pulled off the scrap heap and made prime time like what was done with John Stewart on Justice League. After Xanshi, Katma dying, and his crippling, and creators just not valuing him at all, the character's worth was destroyed, but Justice League turned that all around. Shame the comics didn't capitalize on it nearly as well as they should have.
If you want to talk about comics, his best moment is probably defeating Hal Jordan in Green Lantern: Mosaic #5.
Wrost:
Xanshi. He never recovered from that, and he was written way out of character.