The Foundation is not actually the most powerful GOI in the world. The Chaos Insurgency is. Nobody realizes this, not even the Foundation, because the CI keeps the extent of their power under wraps.
In the current backstory for them, they basically broke off from the Foundation and in the process they stole a lot of SCP's. Likewise, there are basically NO objects that the Foundation has recovered from them, except for those that the Insurgency allowed the Foundation to have.
In my view of them: they stole all the USEFUL objects/individuals.
Cue semi-conflicting headcanon time:
I'd dissent on which is more powerful. I think the Foundation has a lot more bureaucratic and political clout in Europe, North America, and the former Soviet Union. The Foundation is a large, bureaucratic and hierarchical organization with all the benefits and drawbacks that accompany such a status. The Chaos Insurgency, on the other hand, is a non-hierarchical, fluid, cell-based network (as a student of guerrilla insurgency theory I'm inclined to take the "insurgency" term seriously). With that structure, few if any of the members (even the "leaders") of the Chaos Insurgency know the full extent of their resources and agents. With compartmentalized cells, the organization can run extensive and complicated operations with limited risk of exposure should any individual member be captured. The political power of the Insurgency is either what it gains through direct control in Third World countries; through blackmail, sabotage, assassination, and other criminal activities performed for political ends in First World countries; or by getting the Foundation or other GoIs or developed governments to do things for them - where the Insurgency does something not because the act itself is the goal, but because the reaction of the Foundation (or whatever other organization is targeted) is the intended result. Like a skilled martial artist, the Insurgency uses the momentum of its opponents for its own purposes.
In my eyes the Foundation also has more SCPs in their possession than the Chaos Insurgency. The Insurgency did steal the vast majority of the useful ones the Foundation had when the Insurgency broke off in 1924, and unlike the Foundation the Insurgency has absolutely no scruples about using SCPs to suit their purposes. Accordingly, there is a force-multiplication effect.
Basically, as I see it, there are times where Foundation forces come head to head with Insurgency forces, and the Foundation forces come away alright (perhaps even believing they won), but the Insurgency never loses outright. Put simply: The Chaos Insurgency never loses.And if someone writes up a tale where the Foundation hands the Chaos Insurgency their collective ass, I'll probably downvote.