Doorknob. The only explanation I can come up with is there was this time in my childhood when my sister managed to make doorknobs really funny for me. I don't remember what happened, exactly. But the remains of the memory are just enough that I still feel like giggling when I hear the word.
Other words that make me giggle: Parsnip, midichlorians, cockle, figs, penis (ugh, it took all my strength to not smile and giggle at that word every time my son's pediatrician said it at the recent appointment...so mature, I am), and any word relating to poops or farts.