While the Romans weren't the only great empire, with others conquering more territory and faster, and there are a number of great innovations which others take deserved credit for(and most of what the Romans did was not invented by them, their genius was taking older forms and vastly improving upon them). But there are several reasons why the Roman Empire could rightly be considered the most successful empire there has ever been.
1)Longevity, there was a state that identified itself as Roman for almost 2000 years, only China can claim a longer use of the same name(so far as I am aware). More importantly the Roman Empire was the dominant power in Europe and the Mediterranean for 800 years(roughly 100BC to 700 AD), even losing Italy and Rome itself for the last 200 years of that didn't change their dominant status even if they weren't a huge empire any longer. No other empire or Dynasty can claim that kind of longevity of dominance.
2)Related to that, virtually all other empires rise quickly and fall quickly, the Roman Empire rose slowly and fell slowly.
3)Engineering, while not the only great builders and, again, they didn't invent most of what they used to so great effect, but there are a number of Roman engineering achievements that were not surpassed until the 18th century when steel reinforcing came into wide use. They built aqueducts stretching dozens if not hundreds of miles that were carefully built to only descend about 6 inches per mile and provided millions of gallons of water per day, they had running water on the third floor of the coliseum which could be emptied in less than 20 minutes, and the dome of the Pantheon is STILL the largest unreinforced concrete dome ever built(that has never cracked, others were built larger, all of them cracked) and no dome was made larger period until 1850.
4)Law, even the Greeks recognized the Roman preeminence in Law, and as already noted their law codes are still hugely influential around the world.
On the other hand, I kind of agree that Julius Caesar is a bit overrated. He was a great man, sure, and he did extraordinary things, but while a great general, Pompey was better(Pompey lost the civil war in part because while a fantastic general he had been out of the field for 10 years, had an inexperienced army, and, most importantly, he clearly never understood Caesar while Caesar seemed to be able to read him like a book). He was a great speaker, but Cicero was better, he was a great Politician but Augustus was FAR better, and he almost single-handedly destroyed the Republic(although its demise was perhaps inevitable by the time Caesar came along, if it hadn't been him that killed it, someone else would have) and died perhaps because he didn't understand that sometimes form is more important than function(i.e. even if the Republic didn't work and one man rule worked better(and it does, if you have a sane and capable person as the one man, two VERY big ifs, hence why such systems often don't last long and are not preferred by most people) the form of the Republic was still important to people. Most importantly, however, is that while he did great things and was hugely influential what he did, and his influence, pales in comparison to the achievements of his great-nephew and successor Augustus.
