There are so many great ones! Offhand:
Homer: You tried your best and you failed miserably. The lesson is 'never try'.
Homer: Oh, everything's too damned expensive these days. This Bible cost 15 bucks! And talk about a preachy book! Everybody's a sinner! Except this guy.
Homer: No, no, no, Lisa. If adults don't like their jobs, they don't go on strike. They just go in every day and do it really half-assed.
Bart: What's Santa's Little Helper doing to that dog? Looks like he's trying to jump over, but he can't quite make it.
Bart: There's no such thing as a soul. It's just something they made up to scare kids, like the boogeyman or Michael Jackson.
Chief Wiggum: Oh, man, what a day. It's no cakewalk being a single parent, juggling a career and family like so many juggling balls ... two, I suppose.
Chief Wiggum: I hope this has taught you kids a lesson: kids never learn.
Ralph Wiggum: Me fail English? That's unpossible.
These are from memory, so the quotes may not be exact....
Bart passes a message through the crowd at the school... it comes out like this.
"The teachers plan to strike, purple monkey dishwasher."
The TV executives when Homer and Moe spoil the plot of a soap.
"Oh no, they're giving away the plot! Go to commercial.... Right...... [long pause] Now.
Moe with Homer...
Moe: "This fryalator will deep fry a Buffalo in 40 seconds."
Homer: "Awww, but I want it now!"
Golfing, Mr. Burns says to Homer in the sand trap.
"An open face club, man! Use the Sand Wedge!"
Homer drools, "Open face Club Sandwich"
Also to add to this list...
any time Chalmers yells "Skinner!" (especially when he's just saying hello)
Homer, to Patty and Selma - time to take out the trash!! But first, I'll have to ask you to leave.
or the classic(do you know, that this appears in the script as ANNOYED GRUNT ? Strange but true)
D'oh!
That's interesting. If it appears as ANNOYED GRUNT, that explains the episode (with Sherry Bobbins) where the title is Simpsonsuoercalifragilisticexpialli(ANNOYED GRUNT)cious. (*phew*).