Dragon or Rock. Dragon because it's only resisted by one type while Special Rock moves deal the most damage to OU Pokemon and Rock is the only type without an immunity that hits more types SE than NVE.
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I'll say dragon, because pre-6th gen, they have a good selection of good base power + 100% accuracy (dragon claw, dragon pulse), utility attacks that still hit with reasonable power and accuracy (dragon tail, dual chop), and 120 base power overkill moves (outrage, draco meteor), only resisted by one type and with only one ability making something immune to it (wonder guard). Nowadays iirc, there is no situation where a dragon type won't run a dragon stab in competitively except for maybe a rare completely dedicated defense/support set.
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