Dalek Emperor

"Dalek Emperor" — also "Emperor Dalek" or "Emperor of the Daleks" — was the title held by the supreme ruler of the Dalek Empire, commanding the Dalek race and the planet Skaro.

The Dalek Prime
The Dalek Prime, or "Golden Emperor", was the first Dalek mutant to have climbed into a Dalek War Machine, (COMIC: Genesis of Evil) or, in other accounts, was the Dalek who had fired on Davros and made a proclamation on behalf of the other entombed Daleks that they would "prepare, grow stronger, and, when the time is right, emerge as the supreme rulers of the Universe". (TV: Genesis of the Daleks) It proclaimed itself the Emperor of the newly-created Dalek race. (COMIC: Genesis of Evil)

The Prime would continue to hold the position throughout his life, occupying several different casings, (TV: The Evil of the Daleks, The Parting of the Ways) although during and immediately following the Imperial-Renegade Dalek Civil War, it was referred to as simply "the Dalek Prime" by its side of the War — as the opposing side, the Imperial Daleks, referred to their leader Davros as "the Dalek Emperor". (PROSE: War of the Daleks) The eighth, (AUDIO: Restoration of the Daleks) ninth (TV: The Parting of the Ways) and tenth incarnations of the Doctor, when crossing paths with the Dalek Prime again, recognised him as "the Emperor of the Daleks". (COMIC: Defender of the Daleks)

Emperor Zeg
Zeg was an early Dalek scientist who serendipitously discovered the secret of metalert. Having been granted a casing that was invulnerable to all weaponry known to its race, Zeg was effectively invincible and declared himself the new Emperor, hoping to unseat the Golden Emperor. Recognising that both Daleks had a claim to the title, as Zeg lacked provable intelligence but the Golden Emperor lacked provable strength, the Brain Machine ordered that a duel be fought between them to demonstrate who was truly superior. Through guile, the Golden Emperor defeated and killed Zeg, cementing his status as Emperor once more. (COMIC: Duel of the Daleks)

23rd century Emperor
One Dalek Emperor ruled Skaro in 2254. It possessed a bronze-coloured casing with a bulbous head and large cannon-like gunsticks. Allied with Davros rather than opposed to him, the Emperor presided over a short-lived Dalek invasion of Earth which was put to an end by the Seventh Doctor. Shortly after the failure of the invasion, the Doctor was sent to Skaro by the Time Lords to retrieve a sacred Time Ring stolen by Davros, and, facing the Emperor, caused its destruction with his screwdriver. (GAME: Dalek Attack)

Davros
Davros was made the Emperor of the Imperial Dalek faction of the Daleks during the Imperial-Renegade Dalek Civil War. He was encased in a customised Dalek casing with a round dome top which could open and close, his body below his head and shoulders covered in a mass of tubing and cables, implying he had greatly deteriorated physically. He was last seen heading for an escape pod just before his ship was destroyed in the wake of the supernova (set off by the Hand of Omega) that consumed Skaro. (TV: Remembrance of the Daleks)

Later, Davros' increasingly unstable mind caused him to be overtaken by a totally Dalek personality, with him forgetting who he originally was and answering only to the name of Dalek Emperor. (AUDIO: Terror Firma) Still not remembering his name, the Dalek Emperor who had once been Davros began to occupy a casing strikingly similar to the Dalek Prime's, adding an eyestalk to the spherical dome section. (COMIC: Nemesis of the Daleks)

The New Dalek Paradigm's first Emperor
When the New Dalek Paradigm went back to the ruined Skaro, they created a new Emperor Dalek, reusing the shell of the old one, albeit with new paint. His blue colouring suggests he was a Strategist Dalek, had ties to that role or was the mastermind of all of the Daleks' plans for universal conquest. When the Eleventh Doctor and Amy Pond travelled back in time to when Skaro was devastated and before the new Daleks arrived with the Eye of Time, they erased the Daleks' alterations and Kaalann was left abandoned. The new Emperor was never created as a result. (GAME: City of the Daleks)

The New Dalek Paradigm's second Emperor
When the New Dalek Paradigm took control of Earth in 2106, they were led by a new Emperor Dalek. This Emperor was different from the Time War Emperor, as it was coloured purple, was able to form a sphere and its weaponry was much larger. This new Emperor wanted to completely change time by using a piece of the Eternity Clock, and remove Gallifrey from existence; the Daleks would then become the new Lords of Time. The Eleventh Doctor and River Song were able to remove the piece of the Eternity Clock from the Dalek's possession, foiling the Emperor's plans and undoing the Dalek invasion of Earth. (GAME: The Eternity Clock)

The Daleks later established a Parliament which took decisions for the whole of Dalek-kind under the leadership of a Prime Minister. When the Eleventh Doctor and his companions were summoned to the Parliament's ship, no Emperor was in evidence. (TV: Asylum of the Daleks) After the fall of the Parliament, the New Dalek Paradigm's first Supreme Dalek took control of the Empire, flanked by the original Eternal Dalek. (PROSE: Dalek: The Astounding Untold History of the Greatest Enemies of the Universe)

Undated Emperors
A Dalek Emperor composed Operation Divide and Conquer, the plan to send to divide the humans and Draconians in 2540, and gave him a squad of Ogrons to complete his task. (AUDIO: The Dalek Conquests)

A Dalek Emperor tried, who was consequently exterminated by the Dalek Prelature. (PROSE: The Novel of the Film)

Other references
The Sixth Doctor objected to Jack Harkness marrying Callista while posing as him, likening the situation to a marriage with the Dalek Emperor. Jack countered by pointing out that the Doctor was still fighting the Daleks, and used a hypothetical romance with Davros to taunt him. (AUDIO: Piece of Mind)

Invalid sources
A (identical in voice and appearance to the one in The Parting of the Ways, though it is not confirmed to be the same individual) acts as a secondary villain in LEGO Dimensions, where he is voiced by Nicholas Briggs. In the video game's story, he is unaware that some of his Daleks have been recruited by Lord Vortech to aid him in his conquest of the multiverse, but he is aware of mysterious happenings involving cross-dimensional travel, and has acquired one of the Keystones for himself (the Scale, which allows him to enlarge or shrink himself and other Daleks at will). Believing the goings-on to be the fault of the Twelfth Doctor, he captures the game's protagonists, Batman, Wyldstyle and Gandalf, to try and find out the Doctor's plans, but ends up getting shrunk to mouse-size by the Scale at the end of the ensuing boss battle. The iteration used by Davros in Remembrance of the Daleks appears in a DLC level, and the iteration seen in The Evil of the Daleks appears in a secret area in a level pack themed off The Goonies.

Other matters

 * Similar to Dalek Supreme/Supreme Dalek, the terms "Dalek Emperor" and "Emperor Dalek" are used interchangably across media. "Dalek Emperor" is used in The Sound of Drums, while "Emperor Dalek" is used in Remembrance of the Daleks, Nemesis of the Daleks, City of the Daleks and Night Terrors. The Evil of the Daleks simply used the term "Emperor", while the Ninth Doctor identifies the "Emperor of the Daleks" in The Parting of the Ways.
 * Depending on the player's progress, "Dalek Emperor" is a potential rank for the Metaltron to attain in the video game NOTVALID: The Last Dalek.