I've read a few times that the freedom fighters in SE europe were amazing. I'm not too sure who exactly they were fighting (I assume they're referring to Germany, maybe Italy or Soviets later in the war) so clarification of the parties involved would be helpful.
I had previously asked a question about guerrillas in West Europe -- apparently they didn't have effective guerrilla armies. How come this area had good freedom fighters, but W europe didnt?
While you're waiting for experts to answer, you might be interested in reading some previous threads on partisan activity in Nazi-occupied Europe:
Insurgencies
Why are the Yugoslav partisans considered the most effective resistance movement in WW2? - 1 comment, over 9 months old.
The single comment summarizes the large numbers of personnel, the favorable terrain, and the high morale that Yugoslav partisans possessed in contrast with the fewer advantages of other resistance movements.
How effective was the French Resistance during WWII? - 5 comments, over 2 years old.
The commenters primarily address the French Resistance activities leading up to D-Day.
In WWII, what percentage of French fought on the side of the resistance as opposed to the Axis? I've heard that more French aligned with the Nazi's, but would like confirmation and context. - 6 comments, over 2 years old.
The commenters discuss the French political climate during the Nazi occupation to show the amount of effort that would be dedicated to resistance as opposed to other matters.
Are there any specific examples of the French Resistance making use of the catacombs of Paris during World War 2? - 4 comments, over 2 years old.
There are a few sourced answers here worth reading.
Where did the French Resistance stand ideologically? - 1 comments, over 3 months old.
Although only one comment, the comment gives an overview of the ideological motivation for French Resistance members as well as a link to a youtube lecture on the matter.
Looking for a definitive history of Jewish resistance in World War II - 2 comments, over 4 months old.
This is good if you want to know what books to read about one corner of the resistance.
Were there German underground/partisans fighting against Nazi Germany? - 18 comments, over 1 month old.
The top level comment goes into German youth resistance which is a corner not often heard of, as well as some general information about more traditional resistance and why it wasn't very common.
Nazi Counter-Insurgency
How effective was the German strategy of retaliation against the civilian population in the fight against partisans/resistance group during WW2? - 5 comments, over 6 months old.
The commenters gives a survey of retaliation in occupied territories in the Soviet Union and a few areas in Western Europe.
4 questions on ww2 partisans (why eastern europe? is nazi effective? any german partisans? any modern ones?) - 3 comments, over 1 year old.
The users in this thread talk about massacres in Yugoslavia and also the German side of the massacre policy.
English translated primary sources on rape by Red Army OR German counter insurgency tactics in Belorussia - 5 comments, over 1 year old.
A user provides books and articles in English and Russian on the topics above.