Align an object to a specific face I usually face this problem when I cut objects to make them printable in my 3D printer. As you may know, you need your object to have a nice base to be printed.

So I usually use bisect to cut an object to different pieces. As in this example:



and I need to rotate the top part to have a face as bottom:



I'll share my workflow in an answer, but I'm asking for a simpler solution, if there were any.
So this is my workflow:

First select the face you want to be the ground of the object, and create a custom Transform Orientation from the N panel:



Then select the new orientation and place 3D Cursor at the face center with ShiftS, Cursor to Selected. You could also move object center to the same place with  ShiftCtrlAltC>Object to 3D Cursor.



Now add an emtpy and align it to my custom orientation.

    

Now parent the object to the empty (CtrlP), clear the empty rotation (AltR), and clear the object's parent keeping transformation (AltP>...).

 

Finally rotate the piece 180º, and you have the face as the ground.