4. Holmes and Hinchcliffe
When Robert Holmes and Philip Hinchcliffe took over from Terrance Dicks and Barry Letts as script editor and producer respectively, they brought a brand new sense of style and freshness that was needed for Tom Baker's first season in the role.
Despite the fact that Season 12 consisted of stories commissioned by the previous production team, both Holmes and Hinchcliffe instantly made their mark. The stories had a darker tone and delved further into horror, instantly helping Baker's new era stand out from the more colorful era of his predecessor.
But it's perhaps with Season 13 that Holmes and Hinchcliffe truly got to define their era. After a season full of established monsters, Philip Hinchcliffe wanted to focus on brand new horrors. And horror was what both script editor and producer brought to that season, with stories particularly inspired by the British horror film studio Hammer.
This partnership only lasted until the end of Tom Baker's third season, with Hinchcliffe exiting the show after Season 14. But there's no question that the era that Holmes and Hinchcliffe created together is still one of the most fondly remembered eras of Doctor Who to this day.