Which, despite the Memetic Mutation surrounding it, isn't anywhere near Leonidas' Crowning Moment Of Awesome. The fight against the Giant Mook and his response when the Persians demand that they drop their weapons are both arguably superior.
King Leonidas: Persians. Come and take them!
...which is Truth in Television and Older Than Feudalism. Famously, in real life Xerxes offered to make Leonidas king of all Greece if he would join him. Leonidas replied that it was better to die for Greece than to rule over it. Xerxes then ordered the Greeks to surrender their arms, to which Leonidas replied molon labe (the original Greek of "Come and take them"). That is the motto of more than one modern-day Greek army unit, including I Army Corps.
It says something when a phrase keeps being badass for more than 2000 years.
"Our arrows will blot out the sun!"
"Then we'll fight in the shade." Motto of the 20th Armored Division of the Greek Army.
Again, according to the Greeks it is Truth in Television. There's a reason we remember them. Unfortunately, all other crowning moments of awesome pale in comparison to what they supposedly did back then.
"Spartans! Ready your breakfast and eat hearty, for TONIGHT! WE DINE! IN HELL!"
All of them are Truth in Television. Yes, even kicking the messengers into a pit. Truth. In. Television. Herodotus has the Spartans doing all of that in his epic The Histories. The Spartans: Actually that awesome.
The Persians asked for earth and water, symbolic of submission to the Persian King. Athens and Sparta threw the emissaries into a pit and a well respectively, suggesting that they dig it out for themselves. That might be one case where real life is more badass than fiction.
"Molon labe" (the original Greek for "Come and get them!") is the motto of more than one Greek army unit and many gun-rights groups worldwide.
Even Queen Gorgo gets one, stabbing the corrupt senator (who forced her to have sex with him) right in the middle of the senate floor. Then as she pulls the sword out, it slices open his money bag, revealing loads of Persian coins from the bribes he was taking. To that, the whole senate got their awesome moment examining the money and all, from young to old, start yelling "Traitor!"
Also, when Leonidas threw the spear at Xerxes. The very act of hitting in the face an emperor who considered himself a god, making him bleed, and humiliating him before his entire army gave Leonidas a first-class ticket to Elysium.
That was also a fulfilling of the promise "The world will know [...] before this battle is over, [...] that even a God-King can bleed."
Xerxes: "Imagine what horrible fate awaits my enemies when I would kill any of my own men for victory."
Leonidas: "And I would die for any one of mine."
Xerxes: "Consider the fate of your women."
Leonidas: "Clearly, you don't know our women. I might as well've marched them up here, judging by what I've seen."
Xerxes: "[...]if you will but kneel at mine."
Leonidas: "[...]but the idea of kneeling... you see, slaughtering all those men of yours left a nasty cramp in my leg, so kneeling will be hard for me"
Stelios and Astanos fighting off the Persians back-to-back; forty-two dead Persians in that two-minute ownage scene.
Leonidas: "Give them nothing! but take from them everything!"
This is where we fight! This is where they die!
The scene with the other Greeks, when Leonidas explains what his 300-man army can contribute to the Greek force, composed mainly of draftees:
Leonidas: Spartans! WHAT? IS YOUR? PROFESSION?
Spartans: HA-OOH! HA-OOH! HA-OOH!
Leonidas: You see, old friend, I brought more soldiers than you did.
Even though the Persians took the pass eventually, they lost around 20,000 soldiers to an army of about 5,000, led by 300 Spartans. Cut to a few months later, when the Persians faced an army comprising, among others, 10,000 Spartans. Many a Persian soldier knew what it was like to lose bowel control that day.