Which item, spell, ability, or whatever has the most interactions and effects in the game? Not the best or most useful, but just the most ways to combine or use something?
I think The Can Of Grease from Nethack may have the most that I know of. Use it on items to protect them from rust and corrosion, to disarm squeaky floorboards, or on yourself for.... I don't know why. You may end up with slippery hands though which sounds annoying and potentially deadly.