Perhaps down. It's a preposition--Take this book down the road; it could be a verb as in 'You down that drink and I'll down mine'; the sun goes down (adverb); then an adjective in 'down payment'; a noun in the feathers on a duck--how many is that? Five? That just leaves interjection, conjunction and adverb...hmmm...Put that down! That's an adverb and that makes six. And if you said to a dog, "Down!" wouldn't that be an interjection? That's seven. And I suppose 'down' as a conjunction would be impossible for even me with my flexible thinking to contrive.