I would say right around issues 44-48. Cobra had sucessfully pulled off creating their own nation, and was recognized by everyone as a legitimate country. Meanwhile, they still controlled the town of Springfield, and were in the early stages of the Serpantor project, which CC at the time thought would be the perfect soldier (not the perfect leader.) At this point they also had every significant Cobra figure on their side except Storm Shadow. Their power would be diminished with the destruction of Springfield in issue #50, but they would still have a run of sucesses that they would ride to the Civil War.
One could actally argue that the currect, DD version of Cobra is also very powerful.
Cobra had about 2 peaks....But I would say around 1990 the Return of Cobra Commander with his all new Cobra legions,and Cobra really kickedass from 90-91.
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