Maybe the Iconians
I dissagree with the above as the ICONIANS were wiped out by their enemies due to the gateways they produced.
It is more likely that the VOTH in the DELTA QUADRANT is the oldest as their doctorine is over 200 millions year old.
87 Million years ago - The D'Arsay
200 Million years ago - The Voth
1 Billion years ago - The Slaver Empire (TAS)
Do we count the Q continuum?