My earliest Doctor Who experiences were with 4th Doctor, Tom Baker, and Sarah Jane. 1980's America was not a haven for Doctor Who, but I definitely remember seeing Pyramids of Mars, likely on PBS. That got me hooked for a while, but despite the presence of other companions, Sladen was the only one to stick with me.
For me, still, Sarah Jane Smith is the best of the companions, and I am certainly not alone. To the best of my limited Doctor Who knowledge she was with more incarnations of The Doctor than anyone else; definitely appearing with Pertwee, Baker, and Tennant on television, and likely others in radio dramas. In 2007 Sarah Jane returned in The Sarah Jane Adventures, a show aimed at younger audiences, but in my opinion quite enjoyable. It tended to have a lower degree of horror than Doctor Who, and was not afraid of fart jokes. Sarah Jane had set herself up as a guardian of sorts, insuring that aliens that came to Earth meant us no harm. This brought her into contact with races known from Doctor Who, such as the Slitheen, but also races that (as far as I know) had not appeared in Doctor Who.
So, goodbye Sarah Jane. Goodbye Liz.
Sarah Jane and 3rd Doctor, John Pertwee
Sarah Jane and 4th Doctor, Tom Baker
Sarah Jane with K-9 and 10th Doctor, David Tennant
Promo from The Sarah Jane Adventures