When one talks about the worse Generals, and that also from American Civil war, then it off course is one controversial subject. A lot of criticism had been generated, especially for the lovers of American War history.
1. Hugh Judson Kilpatrick:

During the civil war, Kilpatrick raid on the capital of the Commonwealth of Virginia ‘Richmond’ on a totally fake basis i.e. freeing the union prisoners. Col Ulric Dahlgren was died during this raid mission. The General also commanded his cavalry in parades as well as in battle, and he seemed to be just perfect in his work. However, when it comes to General Kilpatrick’s discipline, he was a complete fail. His camps were messy, rowdy and were entrenched with prostitutes.
After the failure in the Battle of Gettysburg because of his awful judgment he once again raid on Richmond in the year 1864 to free the union soldiers and his aim was to damage and cause devastation to its fullest but ended up losing his nerve right at the gateway to Richmond and was recoiled.
2. William S. Rosecrans:

General William was appointed as the commander of the Cumberland. He nearly lost the Battle of Stone’s River and then kept waiting for another six months to appoint an armed force which consists comparatively of a very small force.
He commanded during the Tullahoma Campaign, presenting a very poor and faulty strategy. However, he surprisingly made it because of the radical blunders his opponent made.
Another major cause to his failure was his tendency to micro-organize the mobility of units instead of depending on his chain of command. At last, he was to be blamed disgracefully exiting the battlefield at Chickamauga.
3. Don Carlos Buell:

General Buell had complete four divisions under his command along with the Memphis and Charleston Railroad towards Chattanooga while fixing the railway line. His movement was stopped when the line supply was demolished. General Buell also fell back and was forced to back off after the massive attack of Bragg. Hence Buell was pretty much dissatisfied with his job, so were the authorities. His position was transferred to George H.Thomas in 1862 when he again halted the Confederate invasion.
4. Gideon Pillow:

General Gideon Pillow was withdrawn from his job because of his massive faulty strategies in the Fort Donelson operation. He suffered from this embarrassing beat when he decided to leave the fort and left the great task on General Buckner’s shoulder. Hence 1862, was the end to his career, a fail career.
5. Nathaniel Prentiss Banks:

General Banks had a heavy loss of his corps right in the Shenandoah Valley, where he lost almost 30% of his troops.
6. Franz Sigel:

General Sigel was badly defeated by General Breckenridge at the Battle of New Market after his invasion of the Shenandoah Valley. He was substituted by General David Hunter and was dismissed from his command because of the outstanding cadets from Virginia’s valley. Furthermore, General Sigel was blamed for his ‘lack of aggression’ and later was relieved from his command ship in 1864.
7. Braxton Bragg:

He had poor communication skills with his Generals. The problem was even more exaggerated by his unceasing vacillation. Another failure of General Bragg was the march to Kentucky. Stone’s River attack against William S. Rosecrans was also one of his failures. He as well kept blaming his crops for the loss against the Army of Cumberland.
8. Ambrose Everett Burnside:

General Burnside suffered great defeats at Fredericksburg, also he led ill-fated Mud March, and moreover he had obvious failures at Petersburg. Keeping in mind all these defeats, he was sent to the leave which was never recalled.
9. George Brinton McClellan:

General McClellan was a man known for his over estimated nature and slow movement. He used to announce himself the Savoir of the Union but failed to fight. He made the enemies battle plans but still failed to win. Because of his continuous defeats in Iraq his army was transferred to John Pope’s force. Furthermore, the General had charges on him which the press of that time named ‘Mac the Unready’ and ‘The Little Corporal of Unsought Fields’.
10. Benjamin Franklin Butler:

General Butler is known as ‘Beast of New Orleans’. He was very much disliked in the South that long after the war. Chamber pots with his portrait in the bottom were found in many Southern homes. The major failures in his career which lead to the end to his career were operations in Virginia, Petersburg’s mission.
Butler’s offensive bogged down east of Richmond place called Bermuda Hundred, and he was not capable of achieving any of his allocated targets.
General Butler resigned in November 1865. The matter of ultimate shame for him was that his name was found written on urinals in New Orleans.

Another great source for more informations on the American CV: http://www.civil-war.net/