Which class of Burmese did Orwell despise most of all?

Orwell does not come out and explicitly say "here's the class of Burmese that I despise most of all."  But I think the correct answer to your question is probably the Buddhist priests.

Right at the beginning of the essay, he talks about how annoying it was to be in Burma because so many of the "Burmans" hated the English and liked to taunt them if they could get away with it. He talks about how one would trip him and then everyone else would laugh.

But then he says that the "young Buddhist priests were the worst of them all."  He says that all they ever did was hang around and make fun of Europeans.