I laughed greatly when Dumbledore introduces Slughorn to the students and says that he's going to teach potions.
And then Snape's got the DADA job and Harry says, "No!".
Poor Harry, he was so shocked.
And then I liked very much Ginny attacking Zacharias Smith when he was commentating the match... And Slughorn's first "party" in the train was funny too, how he asked about the connections the students have and when Belby (was it him?) was revealed as not having that good connections, he started ignoring him, he didn't get any sweets... Dear Slughorn, funny little character he is. Or when he scooped Snape out of thin air at a party...