Depends how you define 'word' but in tribute to the great Robert Sherman my vote goes to the one he invented with his brother.
According to some sources, the word has some very fancy linguistic roots:[indent]super- "above", cali- "beauty", fragilistic- "delicate", expiali- "to atone", and docious- "educable", with the sum of these parts signifying roughly "Atoning for educability through delicate beauty." Although the word contains recognizable English morphemes, it does not follow the rules of English morphology as a whole. The morpheme -istic is a suffix in English, whereas the morpheme ex- is typically a prefix; so following normal English morphological rules, it would represent two words: supercalifragilistic and expialidocious. The pronunciation also leans towards it being two words since the letter c doesn't normally sound like a k when followed by an e, an i or a y.[/indent]
More fun to know is that the working relationship of the Sherman brothers was not 'practically perfect'. When Disney Studio drafted them in to save the score of Mary Poppins 'creative differences' ended with them throwing typewriters at each other.