My best practical joke was in retaliation for a joke (which can't be detailed on a family oriented forum) played on me by my best friend in college, a EE who graduated a year ahead of me. I had him convinced he didn't actually graduate, and fearing for his new job.
Getting College of Engineering stationary was the first step, that was easily done because the college had changed its logo and literally threw out boxes of the old stationary. I had actually picked up a bunch of it, including envelopes, to use as scratch paper and to do homework problems on. I then had a secretary where I was working (in Texas) as a co-op student type up a letter I wrote (claiming to be his former academic advisor in the EE department) telling him through a bureaucratic screw up it had been overlooked one of his elective courses had not been accepted/credited when he transferred to UC from Case Western after his freshman year there. The beauty of this was the advisor was a Brit, so I was able to sprinkle in a few Brit expressions/spellings to make the letter even more believable. I mailed it back pre-addressed to my brother and had him drop it in the mail at the UC post office, so it had the correct post mark.
The letter advised him to investigate a three hour social science course he wanted to take at a local college (he was in Rhode Island), and to send the particulars to the advisor. It also advised him the College was obligated to inform his employer of his graduation status, and that, while "we" were sorry for the oversight and any associated inconvenience, we'd work with him to get the problem corrected as soon as he completed the three hour course. "We" also "hoped" his employer would understand.
My friend immediately called his advisor, an absent minded professor type, who said he didn't remember sending the letter, but didn't deny it. He asked him to send him a copy (through the mail, no email in 1979), and while standing in line to copy the letter at work, he finally noticed my lower case initials denoting the person who'd typed the letter. That's when he realized he'd been had. He called me that night, not real happy but relieved. He also made me promise we'd never play anymore practical jokes on each other.