Not sure it was the best but it was superior to anything the US had at the beginning of the war.  The Japanese were prepared with a new design.  The US was not. The British had the Spitfire and the German's had the ME-109, both very good aircraft.  Since neither of these ever went up against a Zero it is hard to know which was better.  The Zero was very good because it was small and very manueverable.  The ailerons were about 2/3s the span of the wing making for a very quick roll rate and thus it's earlier success. Size would limit it's range. Off of carriers the range didn't matter much.  The Zero was also using an older engine design (originally, legally under license from Pratt and Whitney, same engine in US P-47 and others. This obviously a mistake to allow Japan the engine technology.)  That engine limited the Zero's capability at higher altitudes.  Once the US war effort got up to speed, newer aircraft came on line that surpassed the Zero's early advantages.  While it took some time,  the US Navy severely limited Japan's ability to operate off of carriers.  Relegated to land based air fields, the Zero couldn't be used in an offensive role anymore; do to it's lack of range, it could only try to defend it's own airfields.  The US was coming out with more and more powerful engines that resulted in faster, longer range aircraft that performed better at higher altitudes, and could carry more fire power.  About this time, the Navy was getting planes like the Corsair.  The Corsair was very hard to land on a carrier and the Navy gave them to the Marine Corps, who were very successful flying from land based airfields later in the war.  The Army Air Corps had the Lockheed P-38, but it was used in Europe early and took a little while to get to the Pacific.  Taking advantage of strengths, again, the Zero's day's of superiority were numbered.  There were others, I just used these two for examples.  Finally, while never flying in the Pacific arena, the famous P-51 Mustang came into service late in the war and is generally considered the best fighter to see action in WWII.  It is also considered the best propeller driven fighter ever built.  The jets were coming. 

As a wrap-up to your question, the Zero was probably the best fighter in the Pacific early in the war.  It did not maintain it's superiority and by the end of the war, there were several better.