Offensively, Fighting/Ghost (Marshadow) is a completely unresisted combination.
Defensively, Steel/Electric (Magnezone, Togedemaru) has the most resistances, being 11. As for the most immunities (ingnoring abilities), Ghost/Steel (Aegislash), Ghost/Ground (Palossand, Runerigus, Golurk), Ghost/Flying (Drifblim), Ghost/Dark (Sableye, Spritomb), and Ghost/Fairy (Mimikyu) are all tied for three.