In SG-1 we met the Tollan. They never went through a 'Dark Age' after being transplanted from Earth, and are thus at least 200 years ahead of us, technologically.
In their first appearance they demonstrate technology which allows them to pass through solid objects, a palm-sized device which can communicate with people light-years distant, and are accepted by the Nox as apparent equals (one of the Nox travels to earth, unburying their stargate, just to retrieve them).
In their second appearance, they demonstrate the ability to disable all weapons (energy and projectile, though presumably not melee) carried by the Goa'uld and Tau'Ri. They demonstrate that they can suppress the symbiote (allowing the human to speak freely), remove it completely (while keeping both host and symbiote alive), etc. They also demonstrate that they have "Big, honking space guns that shoot down motherships," as Jack O'Neill puts it.
In their third appearance, they get wiped out. By the Goa'uld. The motherships had developed 'new shields' which rendered their Ion Cannons (previously able to destroy these ships in a single shot) completely useless. Over two months later, when SG-1 stopped them from giving in the the Goa'uld demands for phase-shifting bombs (capable of penetrating the iris), they were wiped from the face of Tollana, never to be seen again (in the main series, at least, they may have appeared again in the spin-offs or novels).
Is it ever explained how they managed to get wiped out? Their technology was so advanced that they were completely dismissive of the Goa'uld ("We're aware of them. We don't interact.") and they had obviously not lost all military development capability (the Ion Cannons are evidence enough of that). So it seems completely puzzling to me that they could lose, especially since they seem to have lost to the point of annihilation in the course of less than 24 hours.