Which piece of furniture will have the most inertia and give the furniture movers the most difficulty in moving?

The item with the most inertia is the one with the greatest mass.
It's easy to identify because it's also the heaviest one.

There are a great many other issues that make something
difficult for movers to move, besides its weight.  Size, shape,
handles, wheels, getting it around tight corners, etc.  So we
can't necessarily say that the heaviest item is the one that
presents the biggest problem for the movers.