The Wand of Wonder, just because you never know what its going to do next.
Back in my late middle-school and early high school days, I developed my own Wand of Wonder with different results than the one in DMG. One of my players owned the DMG, and had read its description. Even with the randomness, I still wanted to surprise the player when he found his own Wand of Wonder.
I added powers than only a middle-schooler would appreciate, like the stinking cloud feature making a loud fart sound that could deafen everybody within a certain radius.
I also remember the day one of the PCs used the wand's summon dead cow ability. A group of orcs were chasing the PCs, who were low on hit points and trying to escape, up a flight of stairs. The PC with the wand used it as a last ditch effort. Suddenly, the rotting and stinking carcass of a bloated dead cow appeared over the heads of the orcs. It crushed the orcs up front and rolled down the stairs, forcing the others to retreat.
The PCs fled the dungeon, laughing all the way.