Actually... There was supposed to be an easter egg of Ciri in the corpo prologue...
in the form of a magazine cover
The thread is cool. Ive seen so many easter eggs, but its hard to name one over others.
I think one of the more difficult to find easter eggs that I found myself without help, was the skeleton that was hanging upside down from its feet in a remote ice cave in Skyrim (references Empire strikes back where Luke is caught by a Wampa and put in a similar situation)
If I recall, Skyrim had quite a fair few Easter Eggs in it because I remember the one about the Headless Horseman