The best prank I ever pulled off was with my brother.
Our neighbor was a constant drunk and had one of those huge dually beater trucks for work. He'd often overshoot the concrete curb when coming home and we'd find his trunk over the sidewalk in front of my parent's townhouse at the time.
Prior to this someone bumped into my car in our parking lot. The next morning he parked way far away oddly...we knew he did this.
Another morning a few months later we found he had basically parked on top of the hood of my brother's CRX. We knocked on the door and he came out all biligerant and told us we must have driven UNDER his truck.
Long story short he told us not to call the cops and to get an estimate and he will cover it.
My brother brought one over the next day and he told him to basically get bent and slammed the door.
A few days later we were in my room with our g/f's listening to cell and cordless phones on a scanner. There were two married people in our development having affairs and the weekends were always juicy.
We pick up our neighbor talking to a chick that was apparently both his friend and in charge of payroll. While over her house he saw the pay checks of his co workers and although he was there the longest he was making A LOT less than everyone. He was pissed and was going off on her as if she determined the pay. You can tell he was drunk off his ass. I can't remember her name now, but he kept yelling "NO FUCK YOU JENNY, YOU LISTEN TO ME"...and kept saying his bosses name "THAT PRICK ROBERT CAN GO FUCK HIMSELF PAYING EVERYONE MORE THAN ME!"...she kept hanging up and he kept calling back. We put together her phone number from tones.
We thought it was funny and were about to call her up when the dude dialed information and gave the names then got the numbers for about 3-4 workers. From those calls he got a few other names and numbers. He was going off on everyone.
After he was done we unleashed our prank. We systematically called each person and asked flat out 'why were they discussing their salary and company privledged information?' Most hung up at first and we called back. They wanted 'proof' so we just read off our notes. During the other calls we had pulled up their addresses as well. So many people think you are credible if you can supply their name and address not thinking it's listed.
The story we laid out was our neighbor Mark and a higher level employee (who'd remain nameless) got a bit to drunk one night and he forced himself upon the other guy. Obviously you can't fire someone over that and it would be rather embarrassing should his name come out. 'The company' needs Mark to quit. We mentioned he was also a very dangerous person capable of violence...many people started getting interested in this and even agreeing with a lot of it. The company was international / government contracted and there was a rumor already that they spied on their employees.
We instructed each person not to talk to Mark anymore...if Mark talked to them to ignore him..pretend he didn't exist. We let them know we'd be watching them and could make their employment difficult.
The funny part was many called Mark right back and started yelling at him for 'what the hell did you get me into!!!'. We'd call those people right back and ask them what part of ignoring Mark did they not understand. Most were really creeped out. This was like a Friday night I think...the next day he had a lot of people show up and go off on him...that next day was either a Sunday or a holiday...I can't remember, but starting at 7am we heard all this drilling and sawing. Outside was a 'spy busters' type of truck with a small crew going in and out of his home and car dismantling everything.
When all was said and done they found nothing...they were telling Mark it had to be a cordless interception and recommended he no longer use his phone. 'Chances are someone from his company has been parking within a few hundred feet of his home'.
Long story short....caller id was just starting at this time and we knew we missed blocking one call which we just hung up on. Mark got a call from a friend that let him know the only number they got that was out of the ordinary was a hang up right before they got called by the company....the number was ours.
If we were to get called back one of our g/f's were going to say they must have dialed the wrong number...if pushed the sorry was they were house sitting for their boyfriends parents and just was their to feed the cats. The number was unlisted so if we just bought them off with a likely story we probably could stop further investigation.
What we didn't count on was Mark walking around for hours in the parking lot with a cordless phone dialing our number hoping to catch someone. We had left and one of the girls stayed behind...
Well she decided to turn on the ringer to the phone because she was thinking if we knew we got caught we'd call her so the police wouldn't find her

. She saw his number on caller id and just let it ring and ring and ring...we had turned off the answering machine.
Didn't take Mark long to find our house and realize when he called our phone rang. Still if we were there we could have played it off that we heard the spy story and were trying to call the 'spies' back, but got a worker...
Dumbass never answered the door but came to the window and was bawling and yelling at Mark to 'just go away, we didn't mean it'.
We got home and Mark was there waiting. He told us he was calling the police and that no one at work will even talk to him even though they know the story was false. We told him we had pictures of his various parking jobs including parking on our car...we'd see if DUI was not as bad as a prank call.
He was yelling who the fuck do we think we are and stuff. My brother simply told him pay up or it's not going to stop. I went along with that and I guess the guy realized it was the cheaper way out, the next day he brought over a check and some cash. He began parking away from everyone else's cars too. He moved a few months later.