On the other hand, the question regarding which one is more important to the path is somewhat misleading, since the first four (and some would include the fifth too) precepts overlap with at least three path-factors (right action,right speech,right livelihood).This was said by the Blessed One, said by the Arahant, so I have heard: "For the person who transgresses in one thing, I tell you, there is no evil deed that is not to be done. Which one thing? This: telling a deliberate lie."
The person who lies,
who transgress in this one thing,
transcending concern for the world beyond:
there's no evil
he might not do.
So I think that for the path all are needed, but it is very common that lying accompanies breaking any of the other four, so I think lying can be singled out for extra and special effort and attention.