I don't want to be restricted to online pranks, so I'll tell you about the cruel prank my Geography Teacher played on me.
I'm very studious and we have 2 months till our final forever A2 Level exam.
I got all A's last year and she knows I need an A for University.
We come into the room and shes like "I've been calling the exam board, emailing e.t.c" and we're like "okay? :S"
And she says "read this email :("
We look at the email and it's official from the school Administrator, it basically says that we have only 1 month to our exam instead of the 2 we were told.
Not even I can prepare in that time for an exam, I have another 6 exams I need to study for and one exam that I am actually doing today.
I start freaking out entirely and I'm wondering how to tell the other absent student,
"Look at the date, then tell her" She says.
I'm still freaking out so I am not understanding, she says it three times, I look at my watch and start laughing.

Only prank thats ever been pulled on me, it was yesterday and oh so good.

As for internet pranks, the Dominos Pizza "Put your face on the cheese" was quite good.