My favourite lines would be when the Spartans were "cleaning up" (read: killing the dying) while Leonidas was eating an apple.
Spartan Captain: You know, our Greek friends are dying to have a piece of the action. *stabs a dying Persian on the ground*
Leonidas: *while munching apple* Tell Daxos to gather 20 of his men, able, ready and
sober.
Stringfellow's note 
*Stelios comes running down to Leonidas, out of breath*
Stelios: King Leonidas! *breathing heavily*
Leonidas: Calm down boy, breath easy.
Stelios: *after a moment's pause of steady breathing* The Persian army advances. A contingent. Too small for a charge.
Leonidas: *Looks at his Captain* I leave the company at your charge. *smiling*
Spartan Captain: But, my King...!
Leonidas: Do not worry, my friend. If they assasinate me, entire Sparta will go to war. Pray.....that they are that stupid. Pray........we are that lucky.
*Stringfellow's note: How bloody nonchalant is THAT! Selamba AND awesome at the same time!Leonidas walks up the hill and continues..
Leonidas: *talks with his mouth full munching apple* Besides....there is no reason we cannot be civil about this, cant we?
*Stringfellow's note: THIS is the BEST LINE DELIVERED in the movie for me! Leonidas talks about being civil and diplomatic, yet his act of talking with his mouth full clearly belies his intention of being as "Spartan" as he can be. Funny and fcuk-i-dont-a-damn attitude at the same time! 
*Leonidas throws his remaining apple to Stelios who takes his bite in laughing.*
Spartan Captain: Yes, my King. *stabs another dying Persian on the ground*
That entire sequence is the best in the movie for me. Of course, the action sequences are God-awesome, but when it comes to spoken dialogues, that sequence i described there, together with
"This is blasphemy this is madness.....This is Sparta!" and
"Spartans, prepare your breakfast and eat hearty, for tonight, we dine in HELL!" ranks up there as the most memorable dialogues in the movie for me.

P/S: The "Tonight, we dine in Hell!" line is technically inaccurate. Greeks followling their mythologies would say, "Tonight, we dine in Hades!" and not Hell, but since the movie is refreshed to reflect modern definitions, Hell replaces Hades. There might be those who had no brush with anything Greek mythos, that wouldnt have a clue what Hades is.