The Daleks are considered the greatest and most prolific enemy in Doctor Who, becoming instant hits when they debuted on British TV screens way back in 1963. Dalek mania soon took over the entire country, and later, the world, but it became clear that these homicidal pepper pots would need to evolve in order to stay interesting.
Throughout both the classic and new series of Doctor Who, multiple attempts have been made to single out specific Daleks who were different from the typical foot soldier class. These Daleks ended up changing the trajectory of the species entirely, introducing new plot elements that help them grow beyond their simple, terrifying nature.
The Genesis Dalek
It could be argued that this particular Dalek was the first to exercise its programmed malevolence through its own autonomy. This was the Dalek who decided to break free of its creator's control and declare independence as a species.
Doctor Who's menacing supervillain Davros created the Daleks in Skaro's distant past, but he protested his creation's attempt to exercise independence and tried to bring it back in line. The Dalek responded by murdering all of his staff, before finally turning on Davros, himself. This would trigger not just a long-running battle for supremacy between the Daleks and Davros, but the former's ascension into a galactic threat.
The Dalek Emperor
To begin with, Doctor Who fans weren't sure exactly who was in charge in of Dalek hierarchy. This all changed during the Second Doctor's run when he came face to face with a monstrosity dubbed the Dalek Emperor.
This particular Dalek was encased in a massive metal casing and spoke in a much more intimidating, booming voice than its underlings. The Daleks viewed the Emperor as their undisputed leader - a crucible by which they could carry out their plans of universal conquest, unabated.
The Supreme Dalek
There is still some confusion as to what role the Supreme Dalek plays, and there have been several that have shared the title. It's possible that multiple Supreme Daleks exist, each one presiding over their own group, and answering to the Emperor.
In that case, the Supreme Dalek could be considered the equivalent of an iral who leads his troops into battle, yet still takes orders from higher up. If left without an Emperor to answer to, the Supreme Dalek will act as the de facto number one in charge, carrying out orders based on the Dalek creed.
Dalek Sec
Dalek Sec was a member of the infamous Cult of Skaro, a quartet of Daleks commissioned by the Emperor to operate outside of the Dalek screed. In one of the Dalek's worst masterplans, they hid in the void between dimensions, emerging in the early 21st century to attack Earth. The black Dalek leader took the name "Sec," and led the charge.
Sec changed the Daleks irrevocably when he decided to merge with a human at the genetic level, creating the first known hybrid. While others in the Cult of Skaro exterminated him under the belief that he was an abomination, Sec did succeed in showing what could happen when Daleks were imbued with the human factor.
Imperial Dalek
Between the events of the Sixth and Seventh Doctor's travels, Davros had managed to secure the title of Emperor, driving out the lone Supreme Dalek still in control. Building upon the knowledge he gained while experimenting on Dalek/human hybrids on the planet Necros, Davros began upgrading his existing Daleks for future wars.
This forever changed the Dalek paradigm, as the species was obsessed with racial and genetic purity since the start. Davros grafted bionic limbs onto the Dalek mutants inside their casings, which would later serve as the impetus for other Daleks to skirt their own ideology for the sake of expanding their power.
The Special Weapons Dalek
Living in infamy since the late 1980s is the Special Weapons Dalek, a monstrous creation that seems to fill no other purpose except giving the Daleks superior firepower. In hazardous battle conditions, the Daleks would deploy this unit to obliterate entire swathes of opponents with its supercharged battle cannon.
The Dalek became so popular that it was brought back for the new series on several occasions, mostly for the sake of nostalgia. However, this particular Dalek has yet to squeeze off a round in battle, so it remains to be seen if future stories will grant it an opportunity.
The Lone Survivor
When Doctor Who resumed its story in 2005, showrunner Russell T. Davies decided to throw a curveball to long-time fans by insinuating that the Time Lords and the Daleks had fought each other in a cataclysmic war that spanned the fabric of time itself. Not only did this make the Daleks one of many aliens improved in New Who, but it also paved the way for them to evolve as a species.
In the aftermath, this single Dalek was left. The Doctor encountered it in the early 21st century on Earth, where it was kept as a damaged extraterrestrial relic by Henry Van Statten, an opportunistic multi-billionaire. After repairing itself by absorbing Rose Tyler's temporal energy and going on a killing rampage, it ceased hostilities and took its own life rather than continue on as a tainted Dalek.
The Second Dalek Emperor
It isn't known if the Dalek Emperor shown in the first season of the 2005 Doctor Who reboot is the same as the one from the classic series. However, this Dalek Emperor was far more dangerous than its predecessor, driven by a fanatical religious delusion that ended up giving it a God complex.
The Emperor was keen to wipe out Earth centuries into the future, and for all intents and purposes, it succeeded. A massive Dalek fleet had been lying in wait, gaining strength for hundreds of years by siphoning human corpses and turning them into impure Daleks. The Emperor and his fleet were destroyed by Rose Tyler, who absorbed the powers of the Time Vortex, wiping them from existence.
Dalek Caan
Dalek Caan is the only known traitor to the Dalek race, and its actions were driven largely by prophetic visions it experienced during an emergency temporal shift. Somehow, Caan had flung himself back into the Time War and emerged on the other side broken and mentally unstable, with a doomsday message.
Davros and his new Dalek fleet attempted to wipe out known reality, based largely on Dalek Caan's predictions. What they failed to realize is that Caan had orchestrated events in order to draw the Doctor in, giving him the opportunity to wipe the Daleks from existence once more. Caan proclaimed that he saw the error of the Dalek ideology, and sought to undermine it from within.
Rusty
In contrast to Dalek Caan, the being known as "Rusty" was driven largely by an anomaly that opened up the opportunity for him to see the truth about his own race. He came to loathe the Daleks, whom he believed were engines of pure destruction, and actively worked to undermine them from within in a rare example of a Dalek doing good, rather than evil.
Rusty would live on for millennia, hunted by Daleks who each met their end at his hands. He was not hostile to humans or other races, but he was nevertheless hermitic in nature. His only purpose was to destroy the Daleks with extreme prejudice, and he'd become very good at his job over the eons.