Usually you should make the item a trigger and use OnTriggerEnter in the player or item script (the event is sent to both). But when the item has a rigidbody with Use Gravity checked, it falls through the floor. A solution is to place the trigger in a child of the main object: child an empty object to the item, add a collider (usually a sphere) and set Is Trigger. Modify the OnCollisionEnter to OnTriggerEnter like below and attach the script to the trigger, not to the item:

function OnTriggerEnter(other: Collider){
    if (other.tag == "Player"){
        print(parent.name); // the parent is the item
        var player = other.gameObject; // the player is the "other" object
        // use the parent's name to find the script stacking
        var stacking = GameObject.Find(parent.name+"_model").GetComponent(stacking);
        if (stacking.stackAmount >=1){
            stacking.stackAmount++;
            Destroy(gameObject);
        } else if (Inventory.itemNum < 8){
            player.GetComponent(Inventory).AddItemToInventory(gameObject);
            stacking.stackAmount++;
            // I suspect you should destroy the object in this case too...
        }
    }
}

If it's collecting an item, try using TriggerEnter instead.

Make sure you check is trigger on the item's collision component.