in the dictionary:

Zyxt is listed as the last word in the online Oxford English Dictionary, an obsolete Kentish word that is the second singular indicative present form of the verb see.

Yet

zyzzogeton at least according to Merriam-Webster Online

but

“Several online dictionaries list the word zyzzyva, a tropical American weevil, as the last word in the letter Z.”

So there.

“Zyxt” and “zyzzyva” defintions and facts come from a Dictionary.com Frequently Asked Question.