My first exposure to Doctor Who was the Tom Baker/Liz Sladen adventures. I remember watching them on the local PBS station every day at 6pm just before dinner. The episode in which she was possessed by an alien consciousness and forced to take a crystalline hand to a nuclear reactor in order to help it regenerate. She kept saying “Eldrad must live!” and that became a catch-phrase among my brothers & I.
For reasons that remain unclear, for his first few serials, Doctor #4 was saddled with an additional sidekick named Harry - who had to be the most useless companion ever. He never did anything other than stand behind either the Doc or Sarah, or occasionally stumble over something. I’m glad they got rid of him fast. He even made Adric look good in comparison.
But with all due respect for Sarah-Jane, I do believe that the greatest companion of the original run had to be her successor Leela. Leela was probably conceived by the series producers as little more than an excuse to have the companion character run around in the skimpiest outfits possible. But in the capable hands of the vastly under-rated actress Louise Jameson, she actually turned into a 3-dimensional character.
A lot of the time, the companion had little else to do but ask “What is it Doctor?” and scream whenever the monster went boo. Leela however was an actual foil for the Baker’s Doctor. She challenged the Doctor’s judgements and wasn’t afraid to voice her own opinions, which sometimes turned out to be more accurate than the Doctor’s. She was pro-active, even aggressive about confronting menaces. She could have come off as extremely silly, but Jameson gave her a real dignity and made her a believable person - this despite some of the silliest scripts in the history of the show (instant adult clones of the Doc & Leela - complete with clothing accessories - go exploring in the Doctor’s brain to hunt out a parasite that infected the Doctor through a radio transmission!)