While I agree that a backhand overhead is easily one of the most difficult, we are talking about a shot used maybe once per match?? As for shots that are very commonly used and difficult to execute:
Running topspin backhand passing shot, especially 2h. This is a shot that people have to use often and can be extremely difficult to pull off. An opponent charges in and hits it deep to the backhand, you have to hit this shot on the run, hit with pace and spin it low to maybe cross court. Very difficult not to chip or push this shot back. Venus William does this shot very well.
Another very difficult shot which is seen often is hitting a short ball on the rise or a good half volley. People have a tendency to push this shot back when you put it at their feet around the service line. Players who can strike this ball hard that is shin level or on a fast rise can put the opponent on the defensive instead of waiting to get passed because they pushed back a weak shot that sits up. When I am on the defensive I resort to a heavy spin looping shot, either short if they are coming in or deep if they are still on the baseline. Often this shot puts me back on the offensive for people who can't aggressively charge a ball on the rise.
Another very difficult shot is at net and someone drops a hard hit shot below the net at your feet and you have to touch it back. Players like McEnroe made this shot look easy but anyone who has had to try to touch hit a 70+mph ball below the net knows how difficult this can be, especially with today's stiffer less feeling racquets. I am not talking about pushing back a junk ball which sits up, I mean absorbing all the speed and spin to just touching it over the net. Many pros have difficulty with this shot which resort in balls that sit up allowing the opponent to get it back.