The downhill sled ride to Whoville at the beginning.
If you can listen to one verse of "You're a Mean One, Mr. Grinch" without at least cracking a smile, you must have garlic in your soul.
All of the interaction between the Grinch and Max. All of it.
Especially when the Grinch whistles to start the trip - Max leaps, mid-whistle, like a flying reindeer and excitedly gazes down at the town, eyes twinkling - cut to the Grinch and the best Aside Glance ever.
Most of the visual gags during "You're A Mean One, Mr. Grinch", especially when the Grinch rounds up a bunch of globe ornaments by playing pool with them.
When Max waves before everything goes tumbling down.
The Grinch hating everyone in Whoville in alphabetical order.
The Grinch: Hate, hate, hate. Hate, hate, hate. Double hate. (sees one name in the phonebook) LOATHE ENTIRELY!
Crash-testing the Grinch's sled, especially afterward:
The Grinch: (spending several moments staggering away from what's left of his sled) Airbag's a little slow...(Air bags deploy)...but that's what these tests are for!
When the Grinch is debating on whether or not to go to the Whovilation:
The Grinch: The nerve of those Whos. Inviting me down there - on such short notice! Even if I wanted to go my schedule wouldn't allow it. 4:00, wallow in self pity; 4:30, stare into the abyss; 5:00, solve world hunger, tell no one; 5:30, jazzercize; 6:30, dinner with me - I can't cancel that again; 7:00, wrestle with my self-loathing... I'm booked. Of course, if I bump the loathing to 9, I could still be done in time to lay in bed, stare at the ceiling and slip slowly into madness. (Beat) But what would I wear?
Made even funnier by the fact Jim Carrey improvised this whole bit.
The Grinch arguing with his own echo:
The Grinch: I tell you Max, I don't know why I ever leave this place. I've got all the company I need right here. (shouts) Hello?
Echo: Hello, hello, hello...?
The Grinch: How are you?
Echo: How are you... how are you... how are you...?
The Grinch: I asked you first.
Echo: I asked you first... first... first...
The Grinch: Oh right, that's REALLY mature, saying exactly what I say.
Echo: ...Saying exactly what I say... what I say... what I say...
The Grinch: I'm an idiot!
Echo: You're an idiot... an idiot... in idiot...!
The Grinch: (whispering) Alright fine! I'm not talking to you anymore! In fact, I'm going to whisper! So that by the time my voice reverbarates off the walls, and gets back to me, I won't be able to hear it.
The Grinch's voice: If you utter so much as one syllable, I'LL HUNT YOU DOWN AND GUT YOU LIKE A FISH!! If you'd like to fax me, press the "star" key.
How about when he gets a wake up call by singing the Whobilation song?
His efforts to drown them out are even more hilarious. "Play, monkey! PLAY!"
And then when Cindy comes in, she discovers him bending over, allowing the monkey to bang his head with the cymbals again and again.
And the bit where he's designing an outfit for the Whobilation; he goes to the kitchen and yanks the tablecloth from underneath a pile of metal junk, leaving the pile in place as he leaves. Then he rushes back in and knocks the pile off anyway.
"It's not a dress, it's a kilt! Sicko!"
And what's under the kilt when he rips it off? A garter.
The Grinch: Why, for year after year I've put up with it now. I must stop this Christmas from coming... but how? (Realizes what he just did.) I MEAN... in what way?
"WE'RE GONNA DIE! WE'RE GONNA DIE! I'M GOING TO THROW UP, AND THEN I'M GONNA DIE! MOMMY! TELL IT TO STOP!"
Once the sled becomes stabilized:
Grinch: (exhaling; calmly) Almost...lost my cool back there.
The opening scene where he knocks people over with a marble and wreaks havoc on Whoville. He does stuff like go into the post office and switching and throwing letters into peoples' mailboxes, mostly notices for "jury duty" but you can also hear "blackmail", "pinkslip", "chain letter", and "eviction notice".
The theatrics that the Grinch resorts to trying to scare Cindy Lou Who out of his cave are hilarious. Also, they don't work.
The Grinch jumps off-screen, and then back in wearing a white T-shirt, which he begins ripping apart while screaming.
The Grinch: RUN FOR YOUR LIFE! BEFORE I KILL AGAIN!
And then there's these lines after his theatrics fail: