I have to disagree. I would say the worst student were mechanics. Airplane mechanics not so bad but people who can fix cars really really well could never seem to fly an airplane.

The most enjoyable part of instructing for me was being a examining instructor for a 141 school. Giving checkrides was enjoyable because it is always good to see a person complete something that they have worked on for so long. Also it takes the right kind of person to be able to fail a student and build them up so that they come back.