I think it's fair to say that Caesar is considered one of the great captains of history. It is not uncommon to see him mentioned in the same breath as Napoleon and Alexander, though I have not personally seen Moltke mentioned so often. However, this is a very broad historiographical question and I think it is most reasonable to simply state that Caesar was possibly the best general of his time, irrelevant of whether or not he was the best Roman general ever (unlikely) or as skilled as Napoleon, who mastered a very different and far more complicated style of warfare. I personally would rate him above Alexander, whom I personally think was fairly lucky, but that's another story.

Caesar's military performance was not perfect throughout his career but saw steady improvement over time. He had a respectable military career as a young man and by the time he received his first 5 year command in Gaul (59 BC) he had a good military grounding. His first major battles against the Helvetii, Germans and Belgae showed little inspiration but competent leadership and the ability to think quickly and recover from mistakes. These were very large scale and decisive battles that were typical of Caesar's aggressive style of generalship. The war in Gaul dragged out, as one might expect in a populous, advanced and warlike country, and saw him make a serious strategic error in 54 BC by spreading out his legions in territory that was less pacified than he gave it credit for, resulting in the loss of nearly two legions and two experienced generals. Caesar responded with typical speed and decision, but solid and decisive action by his subordinates, particularly Brutus and Labienus, helped him recover the situation.

By 52 BC, Caesar's impressive string of victories year after year saw the Gallic resistance coalesce for this time into something approaching a national movement, under the talented young noble Vercingetorix. Vercingetorix imposed greater discipline on his army than the Romans had encountered before and the result was that Caesar was drawn into several challenging sieges, the first being a major victory at Avaricum but the second being a humiliating defeat at Gergovia, Caesar's first personal defeat in battle. However, Caesar maintained a strong strategic position and superior army, and Vercingetorix finally made a major error by allowing Caesar to trap him at Alesia. Caesar's construction of a rampart around the oppidum, known as circumvallation (in Latin circum means surrounding, vallare is to fortify or entrench), was a standard Roman siege tactic. However, Caesar exceeded preceding examples of his siege method by building a second line of fortifications outside the initial siege line to protect his army from an expected relief force. This was an incredibly courageous move by Caesar, as it left him with no ability to withdraw if he was overwhelmed by Gallic attacks. However, he set out his defences very well and commanded the defence energetically. His army by this stage was of extremely high quality, and the Gauls were at length forced to surrender. The war in Gaul went on for some time, but at Alesia Caesar broke the back of their resistance, and so it truly was an epic victory.

As we all know, in 50 BC Caesar was drawn completely into Roman civil strife and began campaigning against his internal enemies, beginning with perhaps Pompeius' best generals, Afranius and Petreius, in Spain. Caesar was seriously challenged by these two veteran commanders at Ilerda, but totally outmaneuvered them and by cutting them off from water and forage he forced them to surrender, ending the campaign in a single season.

Caesar next went to confront the great man himself, Pompeius, widely considered one of the best Roman generals of all time, though a bit rusty by this stage. Caesar was checked at Dyrrhachium where Pompey showed that he was still no slouch in the field of war, and he withdrew but confronted Pompey shortly after near Pharsalus. Pompey had a larger but less disciplined army, and his main advantage lay in his enormous cavalry numbers. Caesar cleverly drew these cavalry in and counterattacked them with specially assigned infantry units, then met Pompeius' still-solid infantry line and eventually decided the battle with a conventional flanking manoeuvre conducted by his third infantry line. At some stage Pompey fled the battle and Caesar took his camp. The entire episode, as presented by Caesar, is a tad anticlimactic, and it is possible that Caesar misrepresented Pompey's generalship. On the other hand, as suggested by Syme, Pompey's heart may just not have been in it anymore. It was, after all, a tragic and bloody civil war between erstwhile amici that nobody really wanted.

Having defeated Pompeius, Caesar had won the hardest battles of his career. He faced down most of Pompey's generals in the ensuing years, including his own ex-subordinate Labienus (with a bit of a near miss at Ruspina, perhaps) and won handily with decisive campaigns.

Caesar's reputation as a master of war in his own time is therefore fairly well deserved. His tactics were rarely imaginative, but he was a fast thinking and fast acting leader who utilised the Roman army to it's utmost. He added an enormous amount of territory to the imperium Romanorum in an incredibly short space of time. He made errors, but so did Napoleon. In terms of achievements, he would have stiff competition from Pompey. Whether you would call him a genius is a matter of opinion, but he was certainly one of the better Roman generals of the Republic, and at Alesia and Dyrrachium fought in some of the most complex actions the Roman army had experienced to that time. His immortal fame, however, owes at least as much, and possibly more, to his eventual fate and the decisive role his death and subsequent events played in the fall of the Republic as it does to his legitimately exceptional military record.