Such a rich source of strange beliefs. I could go with any of the ones mentioned so far - but to go back to the one mentioned in the OP: the 144,000.
It was Rutherford who "developed" the teaching in the early 1930s that heaven was reserved for no more than 144,000. Prior to his idiosyncratic interpretation of the 144,000 in Revelation, the organization, under Chuck Russell's tutelage, held the conventional view that all Christians had a calling to go to Heaven, either when they died or during the "Rapture".
Rutherford changed that with the literal stroke of his pen and thus gave birth to the "two classes" of Christian teaching that stamps JWs as a world apart from all other Christian religious cults.