The most important work at site is to have a good "Team Work"
As far as site work is concern all works are important because all work is directly related to progress of work and strength of work.
But synergy, coordination, team work, work and scope clarity etc are most important among entire team.
For any project we need 5 M
Any body can have Money, Machines, Material and Methods same....
But no one can have same Man power, that is team.