It's the original and ultimate spoiler:
Rosebud was a sled.
There, I said it.
Now that it's out of the way, let's talk briefly about the greatest film of all time. Don't take my word for it; Citizen Kane tops nearly every list made by anyone who knows anything about film. Those who don't have it in the top spot are overly-romantic saps who put Casablanca there... and they still put this one at #2.
Citizen Kane is a film a clef of Charles Foster Kane, a media magnate alarmingly similar to William Randolph Hearst. That parallel, not an accident, and the resulting controversy would later be examined in the HBO Film telefilm RKO 281, a title derived from the original production coding for Kane. In fact, legend has it that "Rosebud" was really a nickname Hearst had for alleged-mistress Marion Davies... or, rather, a certain "aspect" of her.
This was the first feature for Orson Welles. Coming off his landmark 1938 The War of the Worlds broadcast, Welles was given an unprecedented deal for a first-time director to develop his own story and to have final-cut approval. Welles was director, co-writer, producer and star of the 1941 picture. A true Renaissance man, he may have even catered the thing, too. I took a course in college [I got a minor in film] devoted entirely to the mercurial Welles; the truth of him may have been even stranger than all the legends.
Citizen Kane was nominated for nine Academy Awards, including Best Picture, Best Director and Best Actor for Welles. He and Herman Mankiewicz won for what was then called Best Writing (Original Screenplay). Unfortunately, it was a box-office failure, no doubt due to the negativity from the Hearst empire, and did not fully recoup its production costs until a 1956 revival. It set a tone for Welles as brilliant but hard to make commercial and profitable and even harder to manage. He never achieved the notoriety he deserved.
So here's the real thing: I have watched so many people watch Citizen Kane for the first time and not get how remarkable it is. I will point out the dozens of revolutionary techniques, shots, angles, story devices, etc., to which they respond "Yeah, but I have seen that before." Yes, yes you have, but that's because they were done first in Citizen Kane and copied by hundreds of filmmakers and in thousands of films since.
My point, in a couple of different ways, is that Rosebud was so much more than a sled.