True, Macharius is indeed a god amongst the Imperial Guard. Basically, Alexander the Great in space. However, I'm not sure about him being better than Horus or Lion, they both are primarchs after all, but who knows, maybe if he were alive Abaddon would be getting his ass kicked on Cadia.
Consider the following, the Lion and Horus were considered the greatest tactical minds of the Great Crusade, but they also had the might of the Great Crusade Imperium behind them. That means Space Marine Legions of hundreds of thousands, a united Imperial Army(which includes a combined Navy/Guard), and a far greater tech level, and it still took hundreds of years to complete the Great Crusade. Macharius, using mainly imperial guard and a few space marine chapters, conquered a thousand worlds in 7 years.