Which Leonidas was the Spartan king, who with his "300", gave his life at Battle of Thermopylae?

Question

There are numerous Spartan kings with the name Leonidas (numbered with Roman numerals), however I cannot locate which "Leonidas" was the great hero, along with his "300" who sacrificed their lives as well.

Answer 1

There aren't numerous Spartan Kings with the name Leonidas. There were 2 only.
The first Leonidas was the leader of the 300 Spartans at Thermopylae.
Therefore your answer is
Leonidas I.

Answer 2

"In the Battle of Thermopylae of 480 BC, an alliance of Greek city-states fought the invading Persian army in the pass of Thermopylae. Vastly outnumbered, the Greeks delayed the enemy in one of the most famous last stands of history. A small force led by King Leonidas of Sparta blocked the only road through which the massive army of Xerxes I could pass. The Persians succeeded in defeating the Greeks but sustained heavy losses, disproportionate to those of the Greeks. A local resident named Ephialtes betrayed the Greeks, revealing a mountain path that led behind the Greek lines. Dismissing the rest of the army, King Leonidas stayed behind with 300 Spartans and 700 Thespian volunteers. Though they knew it meant their own deaths, they secured the retreat of the other Greek forces.

The losses of the Persian army alarmed Xerxes. When his navy was later defeated at Salamis he fled Greece leaving only part of his force to finish the conquest of Greece. It was defeated at the Battle of Plataea.

The performance of the defenders at the battle of Thermopylae is often used as an example of the advantages of training, equipment and good use of terrain to maximise an army's potential, as well as a symbol of courage against overwhelming odds. The heroic sacrifice of the Spartans and the Thespians has captured the minds of many throughout the ages and has given birth to many cultural references as a result."

Answer 3

I cant really help you out with which Leonidas it was, but I can tell you he was the 17th King of Sparta from the line of Agiad. And it was the year 480 BC when the battle happened if it helps.

Answer 4

Plutarch refers to him as Leonides I in his "Sayings of Spartan Women", which might explain why he is usually the "un-numbered" King. If he was the original Leonides he could claim the distinction of having the name alone, sans numeral...