There is no pedant more complete than Tom Buchanan. He is very often making a show of learning in a condescending way. Readers need not look further than the first chapter to discover this:
Civilization's going to pieces, . . . I've gotten to be a terrible pessimist about things. Have you read "The Rise of the Colored Empires" by this man Goddard? . . . Well, it's a fine book, and everybody ought to read it. The idea is if we don't look out the white race will be--will be utterly submerged. It's all scientific stuff; it's been proved. (13)
The irony here is that Tom is trying to look learned and winds up looking utterly foolish. I suppose if I had to pick out the one particular sentence that was the most pedantic of Tom, I would say the second to last sentence would be the best. However, taken out of context, I'm not sure that any sentence in itself could be incredibly pedantic. Although there is another example during the Gastby/Tom confrontation in New York City that comes pretty close:
I know I'm not very popular. I don't give big parties. I suppose you've got to make your house into a pigsty in order to have any friends--in the modern world. (131)
That last sentence is pretty pedantic in itself with all of the condescention that the negative connotation of the word carries with it. Tom is most certainly talking down to Gatsby, citing Gatsby's parties and popularity as evidence of the disintegration of the modern world. What is even better is Nick's following comment which, in itself, proves that Tom is being pedantic:
Angry as I was, as we all were, I was tempted to laugh whenever he opened his mouth. The transition from libertine to prig was so complete. (131)
Quite honestly, I think Nick has pegged Tom just right, don't you? Pedantic would almost be too nice of a word for Tom. Prig sounds more appropriate.