what's the easiest way to fix this door?
You can replace the door or purchase a new skin and glue it on. A buddy of mine has quite a few rentals and when doors are damaged like yours he pulls the pins and knob , lays the door on a pair of saw horses runs a bead of adhesive around the Premier then a x in the center and drops the skin on and clamps until the adhesive is dry. Then he uses a router with a bearing tip to trim the edge and cut the door knob hole. This actually makes the door stronger and usually the skin is thin enough where  trim adjustment is not needed but some times the trim adjustment is needed. It sounds like a lot of work but it can be done in about an hour and is much cheaper than a new door and takes less time than setting new hinges and installing the door knob.