Who says the following in Hamlet: "Something is rotten in the state of Denmark"?
Marcellus is the gaurd who speaks the line "something is rotten in the state of Denmark," giving a whole lot of foreshadowing for the rest of the play. It is important to note that it is not Denmark which is rotten but the "state of," because the problem lies in the political hierarchy or the monarchy, not in the actual populace so much or the other aspects of the country.
It is also important to remember that it was in fact Marcellus and not Hamlet that said the words, most folks automatically assume it is Hamlet since he is so fond of pointing out the problems he sees (or thinks he sees!).