Its Bug/Steel, Water/Ground, and Dark/Ghost (If you meant weaknesses, which is what I think you meant). Bug/Steel (Genesect) has only 1 weakness (4x to Fire), Ground/Water (Swampert and others) (4x to Grass), and Dark/Ghost (Sableye, Spiritomb) (2x to Fairy) all only have one weakness as of Gen 6. Before Gen 6 (XY) Fairy types did not exist so Dark/Ghost had no weaknesses, making it the victor. Because of Gen 6, these 3 dual types are tied for first. The Tynamo line has no weaknesses due to levitate, but that is an ability + typing. The singular type with the least weaknesses is electric, with a weakness to only ground, so it is also tied with the dual types above. Hope this helps!!!