Probably my favourite of them is The Guardian's 1977 San Serriffe advertising supplement, which must rank as one of the most elaborate and thought-out April Fool's Day pranks ever. The only thing they didn't think about was the public response: their offices were flooded with calls from curious holiday travellers and travel agents who refused to believe that this beautiful, unspoiled semi-colon-shaped pair of islands wasn't theirs for the despoliation. (Anybody who knows anything about typography will get the joke immediately: the islands of San Serriffe are called Upper Caisse and Lower Caisse, the capital is Bodoni, and one of the languages spoken there is Caslon.)
Another classic is that perpetrated by Discover Magazine in 1995 with their brief story about the Hotheaded Naked Ice Borer of the Antarctic, which tunnels up through the ice, melts the snow beneath penguins, and devours them. The researched who discovered these creatures was a Dr. Aprile Pazzo, which is a joke not quite as obvious as San Serriffe: "Aprile pazzo" is Italian for "April fool". (A further joke in in the piece is that an explorer named Philippe Poisson was apparently done in by the borers: "poisson d'avril" is the French way of saying "April fool".)
And fictitious Dr. Pazzo got me thinking that in Italian, the plural of "pazzo" is "pazzi", and this sounds just like English "patsy", which is to say a dupe, a sap, a sucker: and therefore "pazzi" must be the source of "patsy". So obvious! The most obvious thing in the world!
You'd think that, wouldn't you? But apparently you would be wrong. I was. The Oxford English Dictionary says the etymology is uncertain, and I guess they'd know, but when you have a word in one language that means exactly the freaking same as a word in another language (plural, but still) and is pronounced identically, the inference is naturally that the words are related. But language is not always so straightforward, I guess. Still, I am going to my grave believing that "pazzi" is the source of "patsy".
Something else I believe is that, like expletive "bastard", "patsy" is specifically a word that refers to men. I don't know why some words have these iron-clad connotations for me: I don't usually attach gender to what people generally think of female-gendered words like "nurse" and "whore", but "patsy" and a few others ("schmuck" comes to mind, although it could be because that's Yiddish for "penis") are irrevocably male in my mind.