Which country had the best artillery

[zu Pferd]

Either France or Russia would claim the best artillery.
Napoleon was extremely fond of his artillery and stated that artillery wins wars. He even had artillery as part of his Old Guard.
However, Russian artillery was not to be trifled with either. The gun-howitzer unicorns, among other things, were a serious menace
and no other nation had an equivalent artillery piece. Russian artillery was also endowed with suicidal bravery. In many cases
it was the Russian artillery that was last to retreat and prevent the destruction of the army.
Adam Zamoyski also makes this claim about Russian artillery in his book Moscow 1812: Napoleon's Fatal March

...the gun-howitzer unicorn...