I'd rather not shoot for the longest words, since place names and scientific terms almost always take the cake -- except for that damned Marry Poppins word, of course.
Anyway, I'll mention some annoying an quirky words:
"Remarkable." I hate this word unless it is used alone as a sentence, perhaps with an exclamation. Generally, it has no meaning. If you said anything, of course it was a remark, and the thing you are speaking of was, accordingly, remarkable.
Careering and Careening. Do we need both?
Uncouth. I hate words with prefixes that have no associated word that does not have the prefix. I will note that "couth" now exists in some dictionaries, with the notation that it is a back-formation from uncouth.
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