find the highest gameobject of stacked gameobjects

Just check all the y values. You can add all the transforms to a list, iterate through and check which has the highest y value. List stackedObjects;

 float highestYValue=0;
 foreach(Transform t in statckedObjects)
 {
     if(t.position.y>highestYValue)
         highestYValue=t.position.y
 }

All you have to do is populate the list.

Alternatively you could probably get the reference as you place the block.