I'm sure that there's a variety of competency between units here, but overall, how much of in impact did these fighters have on the war? Were they a major hindrance on the Nazi's efforts, or did the Wehrmacht succeed in stifling them.
I'm sticking to Western Europe because that's an area I'm interested in, but feel free to comment about partisan success in other regions, along with their logistics and type of operations they carried out.
I may be using the term partisan incorrectly, but in case I'm not clear, I'm referring to freedom fighters, armed covert militias, and that sort of thing.
You're using the term "partisan" correctly.
It would be wrong to say that the Wehrmacht even "succeeded" in stifling them—the forces stationed in France to defend against an Allied attack spent most of their time training, and precious little effort on counter-insurgency. It would be more correct to say that they hardly had to lift a finger.
Liebman's Does Conquest Pay quotes the German armaments minister as quipping "what French resistance?" when asked about the effect of the French resistance on war production, and that was about the impact they had before the Allies started arming and advising them on exactly what to do to support the invasion (mostly by delaying German reinforcements as the Allies attacked).
Resistance membership did go up a lot when Germany started rounding up Frenchmen to send to labor in German factories, since escaping to the wilderness was the only way to avoid being shipped off. News of German reversals of fortune probably also helped, by damaging the aura of German invincibility.