This is one of those movies that I've seen so many times that I can quote most of it. Some of the lines in this movie are just unbeatable. I particularly enjoy watching him stumble and fall while drunk, go out to the fancy restaurant drunk and the part with the moose.
I don't know how many times I have seen this sequence but it's funny every time. From the moment Arthur gets to Susan's Dad's place to the bit with the moose, you pretty much laugh the whole time. I remember watching the out-takes regarding the bit with the moose. It went down just like I'd imagined it'd be like. They were all laughing so hard it was difficult for them to film it.
The late Sir John Gielgud was a wonderful addition to this. His demeanor, his one-liners and the way he handled Arthur were all equally hilarious. It's always a funny moment when he whacks him over the head with his hat or tells him he's a spoiled little ____. I laugh every time I listen to the "I'm going to have a bath" and the lines that follow.