I've done some evil ones
In recent years my best pranks have involved drawing people's heads with a permanent marker when they pass out. One victim spent six days in Japanese jail and another was nearly hospitalised when he discovered all the writing on his head when I wasn't around.

My biggest, most elaborate, most classic prank was done when I was 13. My best friend for that year was Chris, a big white trash guy with a mullet who was, how should I put it, brain-cellularly disadvantaged. I was half his size but I could manipulate him to do anything I wanted. For example, read this story:
b3ta.com/questions/myfirstporn/post70748

But he was creative, I'll give him that. Every few months he would create a new persona for himself. One month he was WWF superstar Lex Luger, "the Nature Boy." Yes, pro wrestling fan is a very good qualifier of the sort of person he was. Next month, he was Captain Kirk. We went on a field trip to an IMAX theatre and when he sat down he remarked "This reminds me of my chair on the Enterprise." He saw me as my science officer and wanted to call me Spook (don't worry, I'm not black). The longest lived persona was Lord Vader.

Once at my Ukrainian grandmother's home, I learnt that "you stupid" in Ukrainian is pronounced "tih durnay." Inspiration stuck. I told Chris that "tih durnay" is Klingon for "Hail the lord." Ultimately I had half the school shouting "You stupid!" at him in Ukrainian, and he'd just smile proudly and say "That's right." It lasted a year, until another friend was mad at me for some reason and told Chris the truth. Chris was mad, and I needed to find a way to squirm out of this one.

So I told him he could be a part of the next prank. I got another word, "shusnak," which is Ukrainian for garlic (thus a very important word to them). I told Chris it was really a horrible Ukrainian swear. And I told Chris that I'd convinced another friend, Greg, that it meant "Nice hair." So I got Greg in on the secret: Chris was going to shout "Garlic!" at him, and he was required to run a hand through his hair and say "thanks." Next time Chris saw Greg, "Shusnak!" "Thanks," said Greg, running a hand through his hair. Chris burst into laughter and took off running, and Greg told me "You better not be lying to me." That was pretty well the end of it around there.
(racetraitor has an alibi for, Fri 14 Dec 2007, 2:54, Reply)