i did love this one:
"I believe whatever doesn't kill you simply makes you... stranger"
and:
The Joker: See, I'm a man of simple tastes. I like dynamite...and gunpowder...and gasoline! Do you know what all of these things have in common? They're cheap!
and this was my favourite:
Gambol: [to The Joker] Give me one reason why I shouldn't have my boy here rip your head off.
The Joker: How about a magic trick?
[pulls out a pencil and sticks it upright into the table]
The Joker: I'm gonna make this pencil... disappear.
[Gambol's thug walks over to kill The Joker, who pushes his face into the pencil and kills him]
The Joker: Ta-daa! It's... gone!
(ok, i know the last one isn't exactly a one liner, but in my defence:

)