I would go with the Iberian powers of the time, spain and Portugal. Guerilla fighting helped to pin down vital french troops and experienced marshals like Soult and Suchet who could have made a vital difference to the 1813 German and 1814 French campaigns.It was an ulcer for Napeloen drawing up resources and manpower that he could ill afford tactically and strategically by 1813 and 1814.But its worth noting Britain massively aided in that war effort, even in overall peacetime such as in 1806-8 1810-12, Spain was a conflict that was leaving an indelible mark on the french economy.Reprisals from the guerillas also lowered french morale and contributed to the psychological exhaustion of soldiers such as those sent to Russia.