Landmarks No.20

Genesis of the Daleks.
Dalek history will never be the same again.
With the Time Lords sending the Doctor to Skaro - to the time of the birth of the Daleks - the seeds of the Time War are sown.
Many see this story as rewriting established Dalek continuity. This is mainly due to the Daleks and Thals giving a different version of the war and the mutation of their respective races - one which makes no mention of Davros or Kaleds.
You can look at it two ways. When I devised a Dalek chronology I placed this story at the beginning (obviously) but then had all the other Davros stories coming right at the end of their pre-Time War continuity. They created an empire, the empire collapsed, and so they resurrected Davros thousands of years later to help them.
You might equally argue that the Doctor's meddling here somehow creates a new Dalek continuity - meaning some of the Davros-free Dalek stories never happened.
One bit of continuity that is shattered is the original creation of the Daleks, as conceived by David Whitaker and Terry Nation for the 1960's comics. Skarosians were small, blue-skinned beings, and a scientist named Yarvelling created the Dalek machines.
Whilst Davros is a memorable creation - a fantastic combination of design and performance - the character will lead to a diminution of the Daleks themselves, reduced to the role of "heavies" in the last few stories in which they appeared. RTD wisely allowed them to re-establish themselves as a force for evil in their own right - holding Davros back for an end of series guest appearance.