Doctor Who: Every Companion Ranked Worst To Best
3. Sarah Jane Smith
Technically cheating with this one, as Sarah Jane Smith was first introduced during the Third Doctor's era in the 1970's, but Sarah Jane still made her mark on the modern era of the show as well.
After leaving the TARDIS in the 1976 serial "The Hand of Fear", Sarah Jane spent the intervening years trying to emulate the Doctor, following in his footsteps and using her journalistic skills to try and tackle more down-to-Earth (literally) alien threats that the Doctor managed to miss. This led to her reuniting with the Doctor, 30 years and six incarnations later, at a school that had been infested with Krillitanes, aliens that were attempting to harness the brainpower of the students so that they can solve the Skasis Paradigm. After the Krillitane were thwarted with the help of K-9, Sarah Jane turns down the chance to travel in the TARDIS yet again, instead deciding to rely upon herself more than her old Time Lord friend, a choice that showed an interesting amount of character development between the modern Sarah-Jane and the Sarah-Jane that always needed rescuing from Daleks made of tin foil and Styrofoam.
Before Sarah-Jane had an official spin-off, an attempt to create one was made in 1981 with the K-9 and Company special, which was intended to launch another TV series, but sadly didn't. The special did, however, prove Sarah-Jane's popularity, further cemented by her inclusion in the 20th Anniversary special "The Five Doctors". Once Sarah-Jane reappeared in the new series, an official spin-off, "The Sarah-Jane Adventures", finally came about, lasting for five seasons and further developing the earth-bound adventures of Sarah-Jane and her friends, with an occasional appearance from the Doctor and some notable villains from the parent show.
Sarah-Jane was charismatic and likeable enough to be loved by multiple generations of Doctor Who fans, and with her detective skills and arsenal of alien gadgets, proved to be a Doctor in human form, saving planet Earth when the actual Doctor could not.