I really don't know which one was my "best" prank, but there were many when I was younger. The most elaborate one didn't have as good of a reaction as I wanted, initially, but was really fun to plan and execute, so I will discuss it:

In college, a group of friends and myself created a "secret society" in which we were wreaking havoc throughout the campus. We never did anything too elaborate, and really only were doing things to enhance our breaking and entering skills, stealth skills, and our observation skills. For our first mission, we decided that we would wake up an entire dorm with a loud "explosion" scaring the residents inside. We did some research, and found the safest component to be Dry Ice and Water in a bottle. We went to the local supermarket, bought 10 lbs of dry ice and a case of bottled waters.

In my friend's backyard, we tested the supplies, and ended up perfecting the "bomb". One sip of water, with a little dry ice, a twist of the cap and a toss.... 1 minute later BOOM, water all over the place, the bottle in pieces, and in a closed area, with no people, it was completely safe, and relatively mess-less.

We targeted the dorm right next to the dorm we were in. The complex was 4 stories tall, with a restroom on every floor. The first floor consisted of the RA, a few empty rooms, and a few other rooms, but no students. The rest of the floors were completely full, and those were our targets.

Equipped with radio headsets, dressed in all black clothing, we snuck out of our dorm at 4 AM, and let ourselves into the opposite dorm. There was a 7 second alarm timer on the back doors, which if they were open for more than 7 seconds a loud alarm would sound. We cracked the lock, and let ourselves in, one at a time, and then positioned ourselves at each floor.

There we decided to do the building in succession, 4th floor first, followed by 3rd, then 2nd. 4 people assigned to each floor, 2 for each set of bathrooms. I was in the last group on the second floor. I was supposed to drop the last bomb, and then leave, being the last person out of the building.

We readied ourselves, and gave the command, floor 4 was finished, as the guys exited through the stairs, when they passed floor 3, they executed the plan. Floor 3 was finished, and as they passed by floor 2's stairwell door, we got the command to toss our bombs. I had a little pouch with about half a pound of dry ice, I took a sip of water, put all the dry ice into it, and then tossed it into the bathroom and took off towards the stairwell. Before I got to the door, my bomb exploded with a loud BOOM, and the building began to shake.

I ran faster than I have ever ran before. I was the last guy out of the building, but the first guy back in our dorm. We stripped ourselves of our clothing, and made it into our rooms just in time. As we were running, we heard the other floor's explosions, and saw multiple lights in the dorm turn on.

We stripped out of our black clothes, hopped in bed, and pretended to sleep as the RA was instructed to search the dorms for anyone awake who could have done such a thing.

We created a pretty big fuss on campus, and was only spotted by a couple who was making out in the parking lot. Luckily we weren't caught, and we continued our society until we left college.

Well. I was in the seventh grade then. Our mid term exams were taking place. The day before our physics exam we received a notice stating that instead of physics, we had biology the next day. There were only three of us as the others had gone home. We were frightened like hell. So we contacted the authority, who informed us that it was a mistake and our timetable was not going to be changed but the eighth grade's. We kept the notices nonetheless. The next day, i arrived really early. There was not any of my classmates. Fortunately one did arrive soon. He was a really good student. I asked him, "which topic is your strong point?". He replied temperature and gas laws. Suppresing a smile i showed him the notice. He looked like he was gonna faint.