* Longest by letter count: This title goes to "pneumonoultramicroscopicsilicovolcanoconiosis", a lung disease caused by inhaling very fine silica dust. It has 45 letters!
* Longest by syllable count: The winner here is "floccinaucinihilipilification", meaning "the act of estimating something as worthless." It has 29 syllables.
* Longest by definition: The word "antidisestablishmentarianism", which is the opposition to the separation of church and state, takes the crown.
So, the answer depends on your criteria!