For myself, she is the first Classic Who companion who I remember clearly. Numerous female fans have written of her importance as a role model for girls, but this was not absent from the way we boys related to her either - being the Doctor's friend, the one who gets to travel with him for fun and danger, was a cause of envy and emulation. Her relationship with the Doctor taught me important lessons about mentoring which I try to implement in my life to this day. As I've commented before, her character really blossomed in the Sarah Jane Adventures when she acquired a group to mentor in her own turn, rather than being saddled with the chinless wonders she was given as comic relief in the first K9 spinoff and the 1990s Pertwee audios.
I've been trying to identify my own favourite Elisabeth Sladen moments. Her two farewells, of course, in The Hand of Fear and School Reunion; her shot at the explosives in Pyramids of Mars; her brief appearance in a bikini in Death to the Daleks; her raising the Queen's consciousness in The Monster of Peladon; her reunion with the Brigadier in Enemy of the Bane; the whole second series of Big Finish's Sarah Jane Smith audio plays. But if you want to listen to a lesser-known jewel of Elisabeth Sladen, in I think her only appearance in the Whoniverse where she does not play Sarah or her double, I strongly recommend you get hold of Walking to Babylon (will cost you a fiver), one of the very first Big Finish audios, adapted by Jacqeline Rayner from a Kate Orman novel about Bernice Summerfield, where she plays the ancient priestess Ninan-ashtammu and does it superbly. There's always more of her material out there if you know where to look.
The BBC faces a tough choice now about the Sarah Jane Adventures, in that apparently half of the next series had already been filmed. In my own view there are only two real options: leave it at the end of Series Four (as Big Finish left her at the end of their sequence of audios), or engineer some teachable moment about Sarah Jane's own demise. The latter is much trickier, yet I somehow hope they do it, and do it well; Sarah Jane Smith and Elisabeth Sladen deserve no less.