I'd say Old Ones or Tyranids for genetic engineering technology.
Also with Old One warp science they'd take the prize as most advanced (though the Eldar may have got close at their peak perhaps)
The Necrons and the Eldar were both bloody powerful at their peak, and I do not think there is enough info on the two to adequately answer who was the most advanced at their peak.
I'd say the Golden Age of Tech human empires were not as advanced as the Eldar empire, which ruled the galaxy while humanity was at its peak.
I do wonder what the Krork/Orks would have been like when they still had brain boys. All that instinctive technoloigcal know how and power, but with a greater sense of purpose and intelligence perhaps? I don't know, but considering what the orks can do now seemingly without effort...
Demiurg seem to be quite advanced, and I wonder if they might have had an empire in the past, and what form that might have taken.
Tau at their current 'peak' are the least technologically advanced I'd say, other than the kroot.