I probably would have said the Watchtower Study too, for most of the years I was in. But toward the end there just got to be such a pattern of doing the same book in the Book Study 3 or 4 times over that I think that one took the crown. In fact, the Book Study was the first meeting I stopped going to. At least the Watchtower had more or less fresh material every week.
On the same note, I used to hate their illustration about repeating material in the meetings: "Well, you repeat the same physical meals, don't you? You might have steak tonight and then have steak again next week, right? You don't reject steak just because you've eaten it before." My usual response was, "Yes, I might eat steak tonight and then steak again next week, but it won't be the same steak."