The Secret

From MIwiki

The Secret of Monkey Island is a fabled and obscured matter and (with the possible exception of the fourth game) never disclosed to the players of the Monkey Island games. There are numerous proposed secrets. Several characters have gone after the secret, including LeChuck and Herman Toothrot, though their quests have since been retconned.

The original title

The title of the first game, The Secret of Monkey Island, is the first public mentioning of the proposed secret and the search of which is given as the reason for LeChuck's demise. Despite the title, nothing in the game actually seems to indicate what the secret is. Since the game makes a great deal over the use of the Giant Monkey Head, it could be considered the Secret, or what exists beneath it: the hideout of LeChuck's ghost ship.

However, this would be unlikely as Ron Gilbert, in his own words, has never told anyone what the secret (according to him) is.

Big Whoop

Big Whoop in the second game is heavily believed to potentially be the fabled secret, as it is the focus of much of the action in Monkey Island 2 and the last two Parts of The Curse of Monkey Island. However, at the beginning of Part V, the player can ask LeChuck what the secret is, LeChuck makes a vain attempt to seem mysterious which causes Guybrush to deduce that he himself doesn't know.

Giant Monkey Robot

The fourth game, Escape from Monkey Island, is heavily indicative that the Giant Monkey Robot is the secret of the Monkey Island, as per the cinematic that introduces it: The real Secret of Monkey Island. Whether or not this is intended to be the definitive answer to the matter remains to be seen.