What Pokemon that have just plain HORRIBLE Sp.def?
Caterpie, Weedle, Magikarp, Igglybuff, Carvanha and Deoxys attack forme all have 20 base special defence, the joint lowest out of all Pokemon. They have the most 'just plain HORRIBLE sp.def'.
Try carvanha, it has one of the lowest sp. defense in every game with a base special defense of 20. Hopefully this answers your question. Hope I helped!