I voted for Dumbo. Primarily, I did so because we have rides that are almost identical both in the park and in other parks, and they don't have anywhere near the popularity. The only explanation can be peoples' inherent knowledge that it's not only one of the 1971 originals, but really one of the 1955 originals. It's a visceral reaction triggered by nostalgia.
I think It's a Small World gets the same kind of nostalgic pull in Disneyland, but the effect of the attraction is not nearly as pronounced in WDW (mostly because of the location and ride exterior).
EDIT: I think the tea cups are in the conversation, too. When I think of my childhood at Disney World (mid to late 70s), I think of the song It's a Small World, the Dumbo ride vehicle (it's iconic), and the Tea Cups ride. All three are uniquely Disney.