Apologies for the long post, but I would just like to add a few things from the necron codex fluff to this discussion.
Age of the C`tan
p24 necron codex:
"It is said that the creation of the star gods took place during the creation of the universe itself... Long before planets had formed and cooled.."
So I would say that the C`tan is the oldest beings in the universe.
Age of Old Ones
p24 necron codex
"the planets eventually gave rise to life which began the long climb to sentience. First to cross the sea of stars was a race of beings called the old ones"
So in my understanding the first sentient space race was the Old Ones.
About the Necrontyr (necrons before servitude)
p24 necron codex
"As the old ones spread across the galaxy, younger fiercer races struggled in their wake. The Necrontyr was such a race"
The necrontyr expanded their empire and started colonizing other worlds, this is how they met the Old ones and the "war in heaven" started
The old ones pushed the necrontyr close to extinction (pushed them back to their home planet)
And then the Necrontyr met the C`tan...
RE: speed of necron ships:
P24 necron codex
(after joining up with the C`tan)
"Armed with weapons of god-like power and ships that could cross the galaxy in the blink of an eye"
PS. keep in mind that necrons don't use warp travel, so they have to travel multiple times the speed of light to "cross the galaxy in the blink of an eye"
Creation of Eldar:
p24 necron codex
"The C`tan now dominated the galaxy. The last bastions of the old ones were besieged and the races they had nurtured became cattle..."
"... Eventually even the Old ones, legendary for their patience and implacability became desperate. They manipulated life into new forms with an even stronger link to the warp..."
"... these included the earliest Eldar,..."
Birth of chaos gods:
p26 necron codex
"The growing pains of the younger races threw the untapped energies of the warp into disorder.... The maelstroms of spirits unleashed in the carnage coalesced into the previously formless energies of the warp"
Creation of the orks:
p26 necron codex
"The old ones brought forth newer creations to defend their last strongholds, like the hardy green skinned krork..."
Granted, it does not say ork, but it would seem safe to presume that the old krorks in the current orks
Who won the war between the necrons and the old ones:
p26 necron codex
"The necrons has been vindicated in their persuit of cold science, and had the undoubted pleasure of seeing the old ones civilization collapse..."
According to me, if you're in a war and your enemies civilization collapses, that count as a win in my book... might be wrong though
Then the suggestion that the Enslavers beat the necrons:
"They (C`tan) would allow the enslaver to take what was left and let the galaxy become a wasteland, the psyker swarm would die away and in time the galaxy would throw upnew life for the C`tan to consume."
So just to summarise:
First there were the C`tan
Second were the Old ones
Third were the necrontyr
Fourth were the Necrons (if you would like to add them as a race, since the creation of the necrons killed off the necrontyr)
Fith were the Eldar
Sixth were the Chaos gods as we know them now
Seventh were the kroks (orks)
Now you might ask were is the precious space mareens and humans in this picture...
well according to page 9 the human race were still swinging on trees during the war of heaven:
"the Mon-Keigh were nothing , They were comical tree beasts, part of the eco-system of their world"
Just my 2c