Lisa is probably my second favorite Simpson family member (after Bart.) And as far as i am concerned, Lisa, like the vast majority of characters on the show, was at her best during the shows golden age. Lisa during the first two seasons mostly came of as some kind of depressed philosopher. She was an interesting character already back then, but she wasn't very dynamic or exciting. But then during the onset of the golden age at season 3, she lightened up a bit and seemed to develop a more forward personality, and also became more concerned with actively trying to change the word for the better rather than just complaining about it. And that is the version of Lisa i am a fan of.
Of course, Lisa, like most characters on the show, took a turn for the worse during the Scully years, and never returned to her goldan age status. Specifically, seems to have become a bit to "full of herself", a bit to confident in knowing that she is always right. But even so, i feel that Lisa's personality didn't degenerate nearly as much as that of many other characters, like Homer and Bart's.