Where did Julius Caesar gain his most important victories?

I think that the place that you are probably referring to
is Gaul.  Julius Caesar is most famous (militarily) for his campaigns in that region of
the Roman Empire.  Caesar fought a series of wars against the Gauls for roughly 8 years
in the 50s BC.  These wars made him more famous than he already had been and they gave
him a great deal of power.

It was Caesar's success in these
wars in Gaul that allowed him to come to power in Rome.  When Caesar was recalled from
his post in Gaul in 50 BC, he brought his army with him, crossed the Rubicon, and
launched the civil war that led to him taking power in
Rome.

So Caesar's conquests in Gaul helped to make him
powerful and famous and so I would say that these were his most important
victories.