The Doctor keeps saying Daleks are the most dangerous alien life form. Is there an existing count of their victims? I don't mean a perfect count. It could be "one entire planet". But what basis does The Doctor have for making this claim? Is it anything more than a bias due to the enmity between the Time Lords and Daleks?
Season 1, episode 6, "Dalek", does a decent job explaining the Daleks and how dangerous they are. From the script (emphasis mine):
DOCTOR: What's the nearest town?
VAN STATTEN: Salt Lake City.
VAN STATTEN: One million.
DOCTOR: All dead. If the Dalek gets out, it'll murder every living creature. That's all it needs.
VAN STATTEN: But why would it do that?
DOCTOR: Because it honestly believes they should die. Human beings are different, and anything different is wrong. It's the ultimate in racial cleansing and you, Van Statten, you've let it loose! The Dalek's surrounded by a force field. The bullets are melting before they even hit home.
It's just not their firepower, defensive capabilities, and technology that they're able to develop, but their mentality, so to speak. Their sole mission in life is to destroy every other living thing that is not a Dalek. As starpilotsix said in the comments, they once came close to destroying everything in every parallel universe just because.
Throughout the history of the Doctor Who stories Daleks have been considered the most dangerous race in the universe. At various points throughout both the classic series and the books, people would mention the fact that the Daleks had wiped out entire star systems and civilisations in order to be the only master race in the universe. While there are races that individually hold more power then a Dalek (The Dæmon Azal for instance). The Daleks sheer numbers, the ease with which they can create more of themselves, the inability to reason or negotiate with them and there ability to develop and evolve means that they are the most dangerous race.
The only other species that comes close to the single minded aim of the Daleks to destroy or absorb all other species are the Cybermen, however Cybermen don't have the technological advances Daleks have. Daleks can utilise and later on build themselves Time Lord technology (In The Chase the Daleks built a time machine to chase the Doctor through time trying to kill him). Where as Cybermen have stayed at pretty much the same level of technology since day 1. In addition during the battle of Canary Wharf the Daleks and the Cybermen faced off with 4 Daleks sucesfully taking on a much larger force of Cybermen with very little damage as the Daleks say they could take on and destroy 5 million cybermen with 1 Dalek
However despite all this in Genesis of the Daleks when the Doctor had an opportunity to stop the Daleks ever being created, having been sent by the Time Lords to complete that very mission, he hesitated, stating that for all the pain and destruction they had caused, their existence had made civilisations who had been at war find peace. He instead delayed their development by about 1000 years. (this is one of my fav Doctor Who stories of all time)
Daleks are extremely dangerous, but not just because of their weaponry. In the episode 'Dalek', for example, the titular Dalek, nicknamed "Metaltron" by Van Statten, shows its intelligence on at least three occasions. One, Metaltron tricks Rose Tyler into touching it so that it can extrapolate her DNA and regenerate itself. Two (this one is more hinted at), Metaltron uses its plunger to type in the code to open a door.
Van Statten: That thing can't possibly guess the code.
Ninth Doctor: The Dalek can do anything. It can calculate over one million combinations per second.
Three, Metaltron uses the fire-control system of the building they are in to kill a group of guards who are shooting at it. It activates the system, levitates, and then fires its 'gunstick' at the water, electrocuting the guards. So Daleks are dangerous because of both their weaponry and their intelligence.
Per "The Apocalypse Element", an audio story from Big Finish released in 2000 featuring the Sixth Doctor (Colin Baker), the Daleks destroyed the Seriphia galaxy. This is described as being 4 times as large as the Milky Way galaxy, home to over six hundred billion stars.
I haven't heard the story, and I don't find specific numbers, but (given the Doctor Who universe) it seems likely that this would involve a large body count.