I would second all the Austen, and all the Pratchett, and all the Adams, and, especially, the scene where Harriet falls in love with Peter in Gaudy Night, not a one liner, but devastatingly effective.
I adore the simplicity of "I, Frodo son of Drogo will take the ring, though I do not know the way".
And although it is in no way a one-liner, I was always cracked up by
.
"We shall build an even stronger house," they said, because they were very determined. Just then they saw a lorry coming along the road carrying barbed wire, iron bars, armour plates and heavy metal padlocks.
"Please, will you give us some of your barbed wire, a few iron bars and armour plates, and some heavy metal padlocks?" they said to the rhinoceros who was driving the lorry.
"Sure," said the rhinoceros, and gave them plenty of barbed wire, iron bars, armour plates and heavy metal padlocks. He also gave them some plexiglass and some reinforced steel chains because he was a generous and kind-hearted rhinoceros. "
From The Three Little Wolves and the Big Bad Pig by Eugene Trivizas.