As Mega_Sceptile mentioned, though no Pokemon with the Normal/Ghost-typing exists, that Typing would've had no weaknesses in Gen 1. Technically this can also be said for Water/Dragon, as in Gen 1, the only Dragon-type Move was Dragon Rage, which does fixed damage.
And as Squaaaaaaaaw mentioned, through Gens 2-5, Dark/Ghost had no weaknesses as well, and let to the lovely Wonder Guard Sableye & Spiritomb. (Fun fact: In Gen 4, Fire Fang could still hit anything with Wonder Guard like normal.)
Those are the only exceptions as far as I know, but if not, feel free to tell me. :V