I'm just going to list off a few of my favorites, and give a few reasons why. Definitive "greatest" is a bit silly, as generals in vastly different circumstances hard to measure against one another. Most powerful defaults to whomever controls the best army.
I like and agree with all previous suggestions, so here are some either not mentioned or that deserve further mention:
Henry V of England- Unlike the Rocky series, the Henry series peaked with V. Nearly winning the throne of France with a severely undersized and undersupplied force is rather difficult.
Phillip 2 (the Revenge of Phillip) of Macedonia, and his rather more famous son. If I were going to try and conquer the known world, I probably wouldn't pick Macedonia as a starting point. These guys did (they didn't have much choice) and it worked. Go figure. Phillip built the force that Lil' Alex put to such fine use.
T. J. (Stonewall) Jackson- I don't know if this guy could have directed a war, but on the field he was a minor deity. Consistently embarrassed his opponents.
Gen. George C. Scott/Patton - Could do more than just drive his troops. He had a pretty fair grasp of how to play offense on a modern battlefield.
MacArthur- Inchon was a rather clever move. He also really looked the part.
Oliver Cromwell- Might've been a bit of a jerk, but one can't argue with the results he got.
David - The historical record's a bit sparse on this guy, but he has arguably the most famous upset victory and single combat as well as a spectacular record in building a minor, threatened kingdom into a respectable regional power.