Elisabeth Sladen was one of the best known faces of Dr Who, featuring in the original series for a prolonged period, returning in New Who and then getting her own spin-off series.
She played Sarah Jane Smith, opposite Jon Pertwee and Tom Baker, and was in The Five Doctors. Sladen was also in arguably the best ever Dr Who serial, Genesis of the Daleks, which introduced Davros and saw some of the best dialogue and acting of the series.
Sladen’s draw as Sarah Jane was a combination of being a very normal sort of person in a rather extraordinary situation, and also marked a shift away from the companion as the aesthetically pleasing woman who screams quite a lot. She also appeared in the show with probably the best of all Doctors, Tom Baker, and was very popular with the audience.
Sadly, her death from cancer comes shortly after the departure of Nicholas Courtney, who died in February of this year. Courtney played Brigadier Alistair Gordon Lethbridge-Stewart, the head of UNIT. Courtney featured with just about every one of the original Doctors (he was introduced as Lethbridge-Stewart with Troughton but played a different role when Hartnell was the Doctor).
UNIT has made a reappearance in New Who, but I think the lack of a Brigadier type character is an obvious gap that ought to be filled.
Sladen and Courtney were both very likeable and played significant characters in the series, and will be sadly missed.