There are tons of funny moments. Ron has funny lines all over the place, as do the twins. But, the funniest moment for me has to be in Half-Blood Prince when Harry catches Trelawney trying to hide an empty sherry bottle in the Room of Requirement.
Harry Potter and the Half-Blood Prince; Chapter 25: The Seer Overheard
And then Harry heard a scream and a crash. He stopped in his tracks, listening.
The noise was coming from a corridor nearby; Harry sprinted towards it, his wand at the ready, hurtled round another corner and saw Professor Trelawney sprawled upon the floor, her head covered in one of her many shawls, several sherry bottles lying beside her, one broken.
Harry hurried forwards and helped Professor Trelawney to her feet. Some of her glittering beads had become entangled with her glasses. She hiccoughed loudly, patted her hair and pulled herself up on Harry's helping arm.
"What happened, Professor?
"You may well ask!" she said shrilly. "I was strolling along, brooding upon certain Dark portents I happened to have glimpsed..."
But Harry was not paying much attention. He had just noticed where they were standing: there on the right was the tapestry of the dancing trolls and, on the left, that was smoothly impenetrable stretch of stone wall that concealed--
"Professor, were you trying to get into the Room of Requirement?
"... omens I have been vouchsafed--what?"
She suddenly looked shifty.
"The Room of Requirement," repeated Harry. "Were you trying to get in there?
"I--well--I didn't know students knew about--"
"Not all of them do," said Harry. "But what happened? You screamed ... it sounded as though you were hurt..."
"I--well," said Professor Trelawney, drawing her shawls around her defensively and staring down at him with her vastly magnified eyes. "I wished to--a--deposit certain--um--personal items in the Room..." And she muttered something about "nasty accusations".
"Right," said Harry, glancing down at the sherry bottles. "But you couldn't get in and hide them?"
He found this very odd; the Room had opened for him, after all, when he wanted to hide the Half-Blood Prince's book.
"Oh, I got in all right," said Professor Trelawney, glaring at the wall. "But there was somebody already in there."
"Somebody in--? Who?" demanded Harry. "Who was in there?""I have no idea," said Professor Trelawney, looking slightly taken aback at the urgency in Harry's voice. "I walked into the Room and I heard a voicce, which has never happened before in all my years of hiding--of using the Room, I mean.""A voice? Saying what?""I don't know that it was saying anything," said Professor Trelawney. "It was ... whooping.""Whooping?""Gleefully," she said, nodding.Harry stared at her."Was it male or female?""I would hazard a guess at male," said Professor Trelawney."And it sounded happy?""Very happy," said Professor Trelawney sniffily."As thought it was celebrating?""Most definitely.""And then--?""And then I called out, 'Who's there?'""You couldn't have found out who it was without asking?" Harry asked her, slightly frustrated."The Inner Eye," said Professor Trelawney with dignitiy, straightening her shawls and many strands of glittering beads, "was fixed upon matters well outside the mundane realms of whooping voices.""Right," said Harry hastily; he had heard about Professor Trelawney's Inner Eye all too often before."And did the voice say who was there?""No, it did not," she said. "Everything went pitch black and the next thing I knew, I was being hurled headfirst out of the Room!""And you didn't see that coming?" said Harry, unable to help himself."No, I did not, as I say, it was pitch--" she stopped and glared at him suspiciously.