In The Movie — "Excuse me, sir!! Do you have Prince Albert in a can?? You do?? Well, you better let the poor guy out!!" The incredibly lame joke isn't what's funny, but Tim Curry's ridiculously hammy laugh.
"Kiss me, fat boy!" also deserves a special mention.
The scene in the movie where Bev first meets the losers club after 30 years and makes out with each one (Including the married Bill) is funny. Though those who didn't read or heard of the novel wouldn't get why but those who have would. Though it wasn't mentioned in the movie it is heavily implied that she had sex with all the members of the losers club. It's confirmed in the novel.
Stan's disembodied head in the freezer sounds like it'd be scary, but then the head starts talking and making jokes. "How's YOUR sex life? ... WHAT'S your sex life?"
Richie: I had a friend whose balls swelled up to roughly the size of the tires on a 1959 Cadillac. I was gonna give him a pair of suspenders and a couple of barrels for his birthday - sort of a designer truss - but they went down before then.
Bill: All put with your customary tact and dignity.
Richie: Thank you, Bill, for those words of support. The word "fuck" was used two hundred and six times in your last book. I counted.
Henry and his boys doing Fart Blasters, which got Beverly laughing despite being terrified out of her mind.
In the book, after Ben meets Bill and Eddie, Bill heads home. Eddie breifly talks to Ben about Bill and his stuttering problem is brought up.
Eddie: Bill, he used to only stutter a little. Now it's really bad. Did you notice that he stutters ?