I'll second the portable hole. Aside from general utility in game, it once played a key role in a character's downtime smuggling activities.
It's not for lack of imagination, but I'm a big fan of +X weapons and armor. I'm a mathy gamer, and they ping the pleasure center in my brain without all the abstract thinking that goes into the creative application of more "interesting" items.
That said, while I've never DMd for nor belonged to a party that acquired one, the Immovable Rod is super high on my cool stuff list.