For me, two are the most overrated generals of the Civil War, one Confederate and one Union.
1) Ulysses S. Grant. Despite the popular belief, Grant was not the victor Boonekemper wants to show us. He committed a good deal of blunders, even in his best moments. For example, he was forced back at Belmont, his flanks almost crumbled before C.F. Smith's arrival at Ft. Donelson, was surprised at Shiloh and Prentiss saved him by standing fast and didn't play an active role in the Iuka- Corinth theater. Yet, he committed his worst errors in Virginia, such as being surprised and flanked at Wilderness, made costly and mostly disjointed attacks at Spotsylvania, was saved due to Lee's illness at North Anna and himself regretted his assault at Cold Harbor. In conclusion, Grant was a good fighter, yet he made many often overlooked mistakes.
2) James Longstreet. Lee's "old war horse" has many cons as a general. Whenever he was left to exercise independent command, in detached duty at Suffolk and against Burnside at Knoxville, he performed lacklusterly wasting time and men. Overall, his deliberateness was his main defect. On August 29, 1862 he refused three times to perform an assault which would probably crush Pope and destroy the Army of Virginia, perfecting the Confederate victory at Second Manassas. Yet, his most serious error was disobeying Lee's order at Gettysburg. Having Longstreet do his duty, the Union lines should have been broken and the Army of the Potomac demoralized in its own home country.
Think about what I have said and tell me your opinions.
Civil War Scholar.