Puts the object on top of the xy grid-floor using an align to axis strategy

align_axis.py

# This work comes directly from a tool available on the SideLabs houdini toolset

bl_info = {
    "name": "Align object to axis",
    "author": "Júlio Rodrigues",
    "version": (0, 1),
    "blender": (2, 80, 0),
    "location": "View3D > Object > Align to Axis",
    "description": "Align object to axis. Useful to put an object perfectly on top of the XY floor plane.",
    "warning": "",
    "wiki_url": "",
    "category": "Object",
}

import bpy
from mathutils import(
    Matrix,
    Vector)
from bpy.props import(
    EnumProperty,
    BoolProperty
)

def main(op, context):
    for ob in context.selected_objects:
        mw = ob.matrix_world
        bb = ob.bound_box
        lower_corner = mw @ Vector(bb[0])
        upper_corner = mw @ Vector(bb[-2])
        translation = Vector((0, 0, 0))

        if op.xAxis == 'M':
            translation[0] = -lower_corner[0]
        if op.yAxis == 'M':
            translation[1] = -lower_corner[1]
        if op.zAxis == 'M':
            translation[2] = -lower_corner[2]

        if op.xAxis == 'm':
            translation[0] = -upper_corner[0]
        if op.yAxis == 'm':
            translation[1] = -upper_corner[1]
        if op.zAxis == 'm':
            translation[2] = -upper_corner[2]

        if op.xAxis == 'C':
            translation[0] = -(lower_corner[0] + upper_corner[0]) / 2
        if op.yAxis == 'C':
            translation[1] = -(lower_corner[1] + upper_corner[1]) / 2
        if op.zAxis == 'C':
            translation[2] = -(lower_corner[2] + upper_corner[2]) / 2
        
        bpy.ops.transform.translate(
            value=translation,
            orient_type='GLOBAL', 
            orient_matrix_type='GLOBAL',
            orient_matrix=Matrix.Identity(3))

        if op.applyLocationTransformation:
            bpy.ops.object.transform_apply(location=True)

def produce_axis_items():
   return [
        ('N', 'Do Nothing', 'Do Nothing'),
        ('C', 'Center', 'Center'),
        ('m', 'Min', 'Min'),
        ('M', 'Max', 'Max')
    ]

class AlignAxisOperator(bpy.types.Operator):
    """Align object to one ore more axes according to its bouding box"""
    bl_idname = "object.align_axis_operator"
    bl_label = "Align Object to Axis"
    bl_options = {'REGISTER', 'UNDO'}

    xAxis: EnumProperty(
        name='X-Axis',
        description='X-Axis',
        items=produce_axis_items(),
        default='C'
    )

    yAxis: EnumProperty(
        name='Y-Axis',
        description='Y-Axis',
        items=produce_axis_items(),
        default='C'
    )

    zAxis: EnumProperty(
        name='Z-Axis',
        description='X-Axis',
        items=produce_axis_items(),
        default='M'
    )

    applyLocationTransformation: BoolProperty(
        name='Apply Location Transformation',
        description='Automatically aplies a location transformation',
        default=True
    )

    @classmethod
    def poll(cls, context):
        return context.active_object is not None

    def execute(self, context):
        main(self, context)
        return {'FINISHED'}

    def draw(self, context):
        layout = self.layout
        
        layout.prop(self, 'xAxis')
        layout.prop(self, 'yAxis')
        layout.prop(self, 'zAxis')
        
        layout.separator_spacer()

        layout.prop(self, 'applyLocationTransformation')


def menu_func(self, context):
    layout = self.layout
    layout.separator()
    layout.operator_context = "INVOKE_DEFAULT"
    layout.operator(AlignAxisOperator.bl_idname, text=AlignAxisOperator.bl_label)    

def register():
    bpy.utils.register_class(AlignAxisOperator)
    bpy.types.VIEW3D_MT_object.append(menu_func)

def unregister():
    bpy.utils.unregister_class(AlignAxisOperator)
    bpy.types.VIEW3D_MT_object.remove(menu_func)


if __name__ == "__main__":
    register()

    # test call
    # bpy.ops.object.align_axis_operator()