Ones where you don't get caught.
Best one from me was making sure my boss got a letter the night of his bachelor party (the night before his wedding), which said that the room he booked for the wedding reception was having to be remodeled.  The letter was post marked months earlier, and it came in an inner office envelope which made it appear to have been misplaced in the mail room for months.  That night he thought that they were not going to be able to have a wedding reception.
He went ballistic, as I had intended, but called his wife and let her know, which was not part of the plan.  She wept all night, I was told.  I was hoping that he would drive down to the athletic club, where the reception was going to be held, and raise hell.  It would have been late and the Athletic Club staff would be scrambling to try to find someone who knew what was going on.  Basically, I wanted him to go off only to find out that everything was fine and he had been pranked.
This was retaliation for pranks that he had played on me and he had insisted that any revenge prank on my part would have to be well planned and original.  Almost a year of planning went into this prank.  Despite that fact that his wife almost had to be committed that evening, he could not criticize me at all for the prank.  You reap what you sow.  Unfortunately, if his wife saw me walking down the street, to this day, she would run me over and back up to make sure that she finished the job.