Perhaps the soviet partisans, that made the greatest efforts and tie down more divisions in the Werhmacht than any other, second could be the partiganis and the resistenza in Italy that practicaly made another war in the north of their country in the last years of the war.
Depends on the definition of successful (Snide but true)
Russian partisans : tied down a LOT of axis troops ion rear areas and most importantly slowed axis supplies to the front by a large factor.
French partisans : Intelligence goldmine assisted with success of D-Day, politically they helped keep France onside in post war era. (By giving the country a belief that they contributed to winning the war.) Without the intel and slowing axis reinforcements after D-Day Russia would have occupied more of Eastern Europe after the war.
Without researching, I would totally agree with @Zinzan and @Markojager and vote for the Soviets. They did more to win WW2 than everyone else put together times 3. The world has never seen war on the scale of the WW2 Eastern Front. Soviet partizans tied down a lot of German troops and were, ironically, then persecuted by Stalin.
However, I know the Chinese also tied down a lot of Japanese forces that could have been facing the US in the Island Hopping Campaign. They didn't tie up as nearly many Japanese troops as the Soviets did German troops, but adding a few thousand Japanese soldiers to each of those islands would made a tough situation even harder for the US.