I said this in another thread as well, but it fits well here.
I'd go with the high EMS Destroy ending from the EC. It makes more sense than a Techno God or that Utopian idea that "We're one big cyborg family."
Since most of the races in the ME 'verse are longer lived in humans, I figured that any future ME games can take place in another galaxy. None of the endings of ME3 said if the entire universe was affected by Destroy or Synthesis. Any of the endings could have been localized to the Milky Way.
I could never see Synthesis being any sort of canon. The implication is way too Utopian for me. Fun as a game ending, but extremely stupid to build another trilogy off of. Control might work, since the Reapers could be just the guardians/army for the Milky Way, but even then I don't want another Techno God become canon. The Starbrat was bad enough.