Unity Editor script to drop objects on the ground using collider, renderer or its pivot point as a base.

PlaceOnGroundEditor.cs

using UnityEditor;
using UnityEngine;

public static class PlaceOnGroundEditor
{
    [MenuItem("Tools/Snap to ground %&D")]
    public static void PlaceOnGround()
    {
        var transforms = new Transform[Selection.gameObjects.Length];
        for(int i = 0; i < transforms.Length; i++) transforms[i] = Selection.gameObjects[i].transform;
        Undo.RecordObjects(transforms, "Place objects on ground");
        foreach (var obj in Selection.gameObjects)
        {
            var collider = obj.GetComponent<Collider>();
            Vector3 pointOnGround = obj.transform.position;
            if (collider != null)
            {
                pointOnGround = GetPointOnGround(collider);
            }
            else
            {
                var meshRenderer = obj.GetComponent<MeshRenderer>();
                pointOnGround = meshRenderer != null ?
                    GetPointOnGround(meshRenderer) :
                    GetPointOnGround(obj.transform.position);
            }
            Debug.DrawRay(pointOnGround, Vector3.up, Color.magenta, 1f);
            obj.transform.position = pointOnGround;
        }
    }

    public static Vector3 GetPointOnGround(Collider collider)
    {
        Vector3 target = collider.transform.position;
        var bounds = collider.bounds;
        var localBoundsCenter = bounds.center - collider.transform.position;
        var originalLayer = collider.gameObject.layer;
        collider.gameObject.layer = 2;
        if (Physics.Raycast(bounds.center, Vector3.down, out RaycastHit hitInfo, Mathf.Infinity, Physics.DefaultRaycastLayers))
        {
            var point = hitInfo.point;
            target = point + Vector3.up * (bounds.size.y / 2f - localBoundsCenter.y);
            target.x = collider.transform.position.x;
            target.z = collider.transform.position.z;
        }
        collider.gameObject.layer = originalLayer;
        return target;
    }

    public static Vector3 GetPointOnGround(MeshRenderer mesh)
    {
        Vector3 target = mesh.transform.position;
        var bounds = mesh.bounds;
        var localBoundsCenter = bounds.center - mesh.transform.position;
        var originalLayer = mesh.gameObject.layer;
        mesh.gameObject.layer = 2;
        if (Physics.Raycast(bounds.center, Vector3.down, out RaycastHit hitInfo, Mathf.Infinity, Physics.DefaultRaycastLayers))
        {
            var point = hitInfo.point;
            target = point + Vector3.up * (bounds.size.y / 2f - localBoundsCenter.y);
            target.x = mesh.transform.position.x;
            target.z = mesh.transform.position.z;
        }
        mesh.gameObject.layer = originalLayer;
        return target;
    }

    public static Vector3 GetPointOnGround(Vector3 origin)
    {
        var target = origin;
        if (Physics.Raycast(origin, Vector3.down, out RaycastHit hitInfo, Mathf.Infinity, Physics.DefaultRaycastLayers))
        {
            target = hitInfo.point;
        }
        return target;
    }
}