It is difficult to exaggerate the impact of Robert Holmes on the series. He wrote 72 episodes, spread across 18 stories as well as being Script Editor throughout the first half of the Tom Baker era.
He introduced Jo Grant and Sarah Jane Smith, The Master and The Valeyard, The Autons and The Sontarans. He was the mastermind who named Gallifrey and then reinvented the Time Lords giving them Borousa and Rassilon. He devised The Key to Time and The Matrix, The White and Black Guardians. He imposed the 12-regeneration limit for Time Lords.
His characters were exquisitely written. Whether petty bureaucrats or megalomaniacs, they lived and breathed thanks to Holmes. Characters such as Sabalom Glitz, Henry Gordon Jago, George Litefoot, Sharaz Jek, Irongron and Pletrac.
Robert Holmes wrote the story voted Best Story of all time in the 2009 DWM readers survey, The Caves of Androzani.
Writing in 2008, Russell T Davies paid tribute to Holmes' legacy,
Take The Talons of Weng-Chiang, for example. Watch episode one. It's the best dialogue ever written. It's up there with Dennis Potter. By a man called Robert Holmes. When the history of television drama comes to be written, Robert Holmes won't be remembered at all because he only wrote genre stuff. And that, I reckon, is a real tragedy.