I'd vote for either the Russian partisans or Tito and his merry band of lunatics in Yugoslavia.
The Russian partisans created havoc behind the German advance, tied down may troops and destroyed/delayed reinforcements and supplies to the front for MOST of the war. Considering the size of this front (and it's importance to the final result of the war) I'd suggest that the partisans probably played as large a role as Lend Lease to the result of the War against Russia. However it gets lower marks as many of the resistance units/leaders were persecuted after the war for being anti-communist.
Which brings me to Tito and his partisans. Whilst he and his band of madmen tied down several divisions of Axis troops they also reconquered (Most off) their country without outside help, they built a political structure within their own country that survived for a further 50 years. Tito was a major instigator of the unaligned group of nation, refusing to kowtow to either East or West and maintaining a stable state in a notoriously unstable part of the world (Just see what happened after his death)
