Chief Dalek was a Dalek rank acknowledged in the Dalek Survival Guide.
The guide identified the Gold Dalek leader of Dalek Invasion Force Type 3 during the Time Paradox Incident as a Chief Dalek. Though it operated a casing they identified as Dalek Supreme Type 3, the guide claimed that it was never actually referred to as a Dalek Supreme and so suggested that it was a sub-commander Dalek, sometimes called "Chief". (PROSE: Dalek Survival Guide, TV: Day of the Daleks) During the Last Great Time War, the Time Lords believed that this Dalek was a subordinate outpost commander who deputised for the Dalek Supreme. (PROSE: Dalek Combat Training Manual) Human historians in the post-Time War universe believed that this Dalek was a member of the Dalek Supreme Council and the Gold Dalek who oversaw Operation Divide and Conquer with the Master. (PROSE: Dalek: The Astounding Untold History of the Greatest Enemies of the Universe)
A Chief Dalek, also known as the "Dalek Leader", was the leader of the Dalek space station and the operation to build a TARDIS fleet to rule the universe with the aid of Shazar. It was distinguished from the Dalek drones by its black dome. The Chief Dalek was destroyed when all TARDIS ships the Daleks built exploded by Cirenium. (COMIC: Return of the Daleks)
Behind the scenes
John Scott Martin operated the Gold Dalek in both Day of the Daleks and Frontier in Space episode six, being credited as "Chief Dalek" in Radio Times on both occasions.