Favorite Magic Item
The Machine of Lum the Mad.
(In my own campaign world, this ancient and dangerous artifact has been hidden and rendered largely dormant in a coffee bar — “Joe’s Mud-Puddle Cafe” — in the city of Mooncastle, where it now functions as the espresso machine of Lum the Mad. True story.)
If we’re not counting artifacts, bag of holding, because every edition of the game has specified what happens if you accidentally tear one, but doesn’t specify how tearable it actually is. Plus, even the smallest bag of holding can be filled with 250 pints of oil (that’s by the weight limit; its volume limit is staggeringly higher, but oil is heavy). Dump that out from a height and follow it with a lit torch, then spend hours arguing with the DM over whether it does or doesn’t do 250d6 of fire damage.
(Archive post from the personal blog.)