I've been beaten to it, but the best word to me is supercalifragilisticexpialidocious - from
Mary Poppins, for those who don't know it.
Oh, and also "moo". It's such a useful word. You can tell someone to "moo off" and you are still being polite, you can enter someones house and greet them with "moo!" (We usually do!), you can randomly change many verbs with moo - "Shall we moo off then?" "Let's moo!" "Time to moo". "Moooo!" (Yes). "Mooo..." (No)
It's great!