Best way to access a control on another form in Windows Forms?

Instead of making the control public, you can create a property that controls its visibility:

public bool ControlIsVisible
{
     get { return control.Visible; }
     set { control.Visible = value; }
}

This creates a proper accessor to that control that won't expose the control's whole set of properties.

I personally would recommend NOT doing it... If it's responding to some sort of action and it needs to change its appearance, I would prefer raising an event and letting it sort itself out...

This kind of coupling between forms always makes me nervous. I always try to keep the UI as light and independent as possible..

I hope this helps. Perhaps you could expand on the scenario if not?