I'm sure it's up for debate how practical an oversized 20 sided die is, though it's created a lot of conversation in the office here and has brought out all of the tabletop roleplayers to be interested in 3D printing - showing again that there are many crossovers in the interests of technology and traditional interests and gaming.
What's your most practical 3D print?
Some times the really simple boring ones are the most practical, such as a plug to cover a friend's rotary washing line. It was just a couple of discs of a certain size stuck together. Quick, easy, useful.
My most recent "practical but interesting" one was a dashboard switch blanking plate for my van, but with just the right sized hole to stuff in a USB port. This, of course, justified the purchase of a new 3D printer - an AnyCubic Photon resin printer to get a nice surface finish. 
