Belisarius - I read somewhere that he never suffered a defeat. He was the master of defensive warfare, taking a secure position that the opponent needs, and forcing the enemy to attack you. If Justinian didn't yank him around all over the Mediteranian, and allowed Belisarius to completely conquer either the Western empire or Persia, who knows how world history would have turned out.
Scipio Africanus - our current military strategy in the war on terror isn't much different from Scipio's. Instead of taking on Hannibal in Italy, Scipio insisted on taking the war to Carthage, just like we want to take the war on terrorism to the hotbed of terrorism, the middle east. Change the direction of the Mid-east, you protect yourself from radical Islamic terrorism.
Themistocles - Had the foresight to prepare Greece for the second Persian invasion, at a time when the Greeks were celebrating the victory at Marathon. Built Greece's navy, used disinformation to lead the Persians into the narrow traps of Salamis that destroyed the Persian navy. Without Themistocles, the Persians would have defeated Greece.
Nelson - I forget any specific tactical advances he made, but I remember during the Tripolitan war between the U.S. and North Africa, the barbarians stupidly raided some British boats, and tried to hold the Brits hostage. Nelson had none of this, swung his fleet down to North Africa, and from ashore pounded one of the cities until the Tunisians/Morroccans not only released the hostages but paid Danegeld to Britain instead. Plus, there wouldn't have been a Waterloo if Trafalgar didn't precede it.
Gustavus Adolphus - First standing armies, at least since Rome and/or Sparta. Anyone who knows Adam Smith-onomics recognizes the value of specialized labor, and Adolphus had soldiers who specialized in soldiering, as opposed to men who were blacksmiths or farmers or lords called up from the inefficient economic system of serfdom to fight battles, poorly. Heck, you could see some of the development of capitalism in Gustavus' contributions to the history of warfare.
And although they weren't the "greatest":
Stephen Decatur - One of the leaders in naval revolution in the early 19th Century, speed trumps size.
Orde Wingate - Helped organize the first Israeli fighters in pre-WWII Palestine. He instilled the idea of going to the heart of the enemy when they least suspect it, night raids in the heart of Arab neighborhoods, to destabilize the confidence of the Arabs and create both confidence in one's own men and an aura of invincibility in the eyes of the enemy.