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For me it's got to be the Goa'uld.
Let's face it the Tollans, the Nox, and the Asgard given the oppertunity would all stay @ home quietly minding their own business if it wasn't for the Goa'uld.
Chances are that the Ancients and the Furlings (Haven't met either race yet but perhaps soon) are of the same inclanation.
The only thing that really seem to fire them all up is the Goa'uld, they just like causing problems, their socilal structure is apparently similar to that of Barons where they are internally jossling for power. And they are opportunists, if they think they can get away with it they will try e.g. the attack on the Tollan home world.
They are a powder keg... and remember old Apophis is out there ... Somewhere
with Sokar's old army. What was it Selmak (Jacob Carter) said "It's ten times larger than we thought."
They are going to be a pain in the Nic'da, but I suspect the Asgard will help us when necessary.
The most important Race to the series - The Goa'uld.
The Most important race to Earth - The Asgard.
Later,