i think general george s. patton is the greatest leader.
Impossible to know. There have been many good generals, but it is impossible to directly compare them as they have conducted different campaigns from each other and during different time periods.
However, a list of great generals would be possible, just not a list of the "Greatest" general.
For example, the Duke of Wellington, Alexander the Great, Napoleon, and Bernard Montgomery would be on such a list. The greatest Australian general of world war one, Sir John Monash might be on it. The problem is that it is only possible to suggest names when they are involved in "great" campaigns and their names become known, many of the best generals, possibly better then the ones listed may exist but remain unknown due to their skills being unknown or them losing a war due to outside factors such as technological inferiority.
For example the best known Australian General presently living is General Peter Cosgrove who was CDF from 2002-2005 and is best known for his handling of the East Timor situation in 1999, may be a better general then General Petraeus in the US but we will never know as they have never been put into the same situation.