A few comments.....Caesar's key mistake wasn't claiming too much power too fast.....rather, it was not bringing his armed bodyguard with him to the Senate on the Ides of March. <g>
Contrary to popular belief, Hannibal was perfectly capable of conducting sieges. His problem wasn't siegcraft or lack of siege weapons; he had engineers and they could have made any weapon he needed. His problem was simply the size of his army.
He's wrongly faulted for not laying siege to Rome after the Battle of Cannae, for instance. But that would have meant trying to lay siege to the largest city in Italy, both in area and in population, with an army of less than 50,000 men, a task hard enough to do in any case. But he'd have to do it with almost all of Italy raising new forces.
He had enough force for a siege, or for a battle, but not for both--yet any attempt at laying siege to Rome would have guaranteed his army would be both tied down in one spot AND would be the target of relieving armies. To fight the relief forces would mean he'd have to lift the siege, which would mean the city could be resupplied, etc. Further still, in those days, the surest way to cause your army to be decimated by disease was to mount a siege, which would have caused his already small force to grow smaller.
Hannibal pursued the only strategy that had any reasonable chance of success. Defeat Rome repeatedly in the field, and try to cause its alliance with other Italian states to fall apart through their defection. Though it failed, against any state of that time except Rome, it would have succeeded. Only Rome had the institutional strength to maintain loyalty among the most of the peoples it had conquered under the defeats that were inflicted upon it.
Now, for worst generals.....In no particular order of awfulness, I offer the following:
1. Benjamin Butler, who, though he outnumbered Beauregard by some 4 to 1, managed to get himself defeated outside of Richmond in 1864, and was then locked up by Beauregard inside a fortified position at Bermuda Hundred. Grant said it was like he was corked up in a bottle. He's also the genius who refused Lincoln's offer of the 1864 spot of Vice President on the Republican ticket. He said he wouldn't take it unless Lincoln could guarantee he'd be President within 90 days of being sworn in. Since Lincoln was sworn in as President on March the 4th, and assassinated on April 14th, Butler demonstrated a wonderful inability to see the future.
2. Charles XII of Sweden. He was one of the most incredible leaders in history. He inspired his men like few commanders ever have. He was a tactical genius, with an incredible ability to take in a given tactical problem at a glance and then instantly order the correct response needed to solve it. And he had all the strategic ability of a rock.....which ability directly caused Sweden's reduction to the status of a minor power.
3. Eric Ludendorff. The man who invented "Total War" and coined the term. The man who demanded that Germany launch unrestricted submarine warfare in 1917 and threatened to resign if that wasn't done, despite being warned it would provoke the US into declaring war; he said the war would be over before the US could intervene. Despite all his undoubted ability, when push came to shove, in the Kaiser's Offensive of 1918, he ignored the strategic principle of reinforcing success in order to follow his plan. By sending reinforcements according to the plan, rather than to the army that was making the best progress, he personally killed any chance he had to gain the peace he launched the offensive to win.
4. Shah Muhammed of Kwarizm. First he needlessly provoked a war with Genghis Khan, by refusing to return the riches pillaged from a Mongol caravan by one of his governors and punish the governor responsible for the crime. Further, when Genghis Khan politely asked for this to be done, he killed the ambassador who made the request, and sent the other members of the embassy back with their beards shaved (Genghis did punish the governor; since he liked gold and silver so much, the molten form of it was poured down his throat).
Then, when Muhammed got the war his actions caused, instead of concentrating his forces, he scattered them all over his empire as garrisons, so that when the Mongols invaded--from 4 different directions!--he had no effective field army to fight with. The end result was that the Kwarizmian Empire was destroyed in one campaign.....though it was considerably stronger in terms of population and troops than the Mongols.
5. Frederick the Great of Prussia. A man of incredible talents in many fields, not the least of which were greed and overconfidence. First he breaks his own pledged word and steals Silesia from Austria.....thus needlessly making a mortal enemy of Austria's Empress. He liked battles so much that he managed to nearly destroy the Prussian Army through the number he fought, a considerable percentage of which he lost through his own overconfidence and the sheer size of the enemies he made through his own greed.
Though he won some incredible victories because of his tactical genius and his army's training, the ending of the 7 Years War found him holed up in a fortified camp because he didn't dare risk a battle in the open. The only thing that saved him to be able to enjoy his honorific title "the Great" was the fact that Russia's Empress Elizabeth--who hated him almost as much as did Maria Theresa of Austria--died, and her successor admired Frederick so much that he declared peace, almost at the moment that Prussia was fated to become a 2nd-rate power again.
6. Braxton Bragg. An interesting man, and possibly the best trainer of troops in the Confederate Army, but one whose talents were definitely not for field command. He commanded at one of the Confederacy's greatest victories--Chickamauga--and at several of its better defeats. He showed considerable ability at planning and executing a campaign, but seemed to run into a mental block when faced with a battlefield. And when he actually WON a battle, he he seemed dumbfounded and totally unable to think of what to do next. After Chickamauga, a rapid pursuit would certainly have led to the capture of Chattanooga, and probably the destruction of the Union forces there, which same were utterly disorganized and demoralized. But instead of pursuing, he sat down for two days and buried his many dead.
His greatest claim to combative ability was his incredible genius for arguing with his subordinate officers, to the point where none liked him, most distrusted him, and almost all organized a cabal behind his back to get him removed on the grounds of demonstrated incompetence. Fortunately for him, he was an old friend and colleague of Confederate President Jefferson Davis.
He cost himself the loss of by far the best cavalry commander in the Confederate Army, Bedford Forrest, by twice stealing Forrest's command and giving it to another man. He got Forrest on his case to the point where Forrest told him that if he ever interferred with him again, Forrest would kill him, and that in the future he would refuse to serve under him. Given that Forrest was a man of his word who killed 31 men in personal combat during the war, this was a promise, not a threat. Bragg never bothered Forrest again, one instance where he showed wisdom.
During his tenure in command of the Confederate Army of Tennessee, it became the best retreating force ever seen in North America.
7. Bernard Montgomery. His failure to destroy Rommel in the "pursuit" after El Alamein proved his incompetence. Though he KNEW he outnumbered Rommel many to one, in men, tanks, guns and planes--through Ultra intercepts of Rommel's returns to his higher HQ--he refused to either launch any kind of a hasty attack against any position Rommel took up. He also refused to try to cut Rommel off, either by a deep flanking operation such as O'Connor used against the Italians, or by a seaborne landing deep in Rommel's rear. By the time he got prepared to attack, Rommel simply withdrew, after his men had had a nice rest.
His ego probably caused WW II in Europe to continue into 1945, by his ignoring of his specific instructions to capture both the port of Antwerp AND the approaches leading to it from the sea--even though he could have done so with no resistance from the Germans and further cut off several German divisions by doing so. The result was that the supply problems which caused the Allied offensive to stall--despite a lack of organized German resistance--continued, allowing the Germans to recover from the defeats in France. He was intent at the time on launching Operation Market-Garden, which was designed to gain for him the glory that had been going to Patton and seems to have ignored pretty much everything else.
It's more than symbolic that in his revision of the British Army's infantry manual, FSR I, he removed the chapter that covered pursuit and exploitation after battle, and kept it removed even when Liddell Hart noted the lack and pointed it out to him. Like Braxton Bragg of the Confederate Army, he knew how to win a battle, but had no idea what to do with the victory.
There are more that I could name, but each of the above is outstanding in his own way.