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. 
I originally became interested in 3D printing to assist in the building of robotics projects ans simple cases for various projects. So far, here are some samples of these types of projects:
Gene
Ive gone through over a dozen spools of filament so picking thwe most practical is hard....
Most useful ones:
crosshair sight for a laser cutter.
https://commons.pratt.edu/open-source/co2-laser-crosshairs/
or a repair cap for a dropped fuel pump I use to drain fuel from yard equipment.....
https://commons.pratt.edu/open-source/3d-printing-a-cover-for-a-broken-honda-fuel-pump/
I print half a dozen items on a slow week.......
Enclosure for MiniZed.
You ask the hardest questions...
The 3D printer has really liberated me to design many useful items that cannot be bought. My 3D print directory has over 1000 files in it in 17 subdirectories and many more sub-subdirectories. These include both design files and print files but it is still a lot to choose from.
The one I will share is a quick-connect adapter for my camera. The quick connect that comes with the tripod covers the battery and SD card compartment, so it must be removed to access either the battery or the memory card. It also does not sit under the center of gravity of the camera, so setting the camera down without the tripod, causes it to tip over.
The custom 3D print tripod adapter solves all of these issues and never needs to be removed from the camera....
Custom quick connect tripod mount Original quick connect tripod mount
In the second picture the camera lens is tilted over, pointing down and resting on the table - I had to move the mirror off the table just to get the shot. I had to put the mirror in front of the lens so it doesn't look like it is pointing down. The really important feature is the new mount provides access to the battery and memory card while the camera is on the tripod.
Like Fred27, my most practical designs are simple and boring.
For example, a clip to keep the kitchen trash bag from falling into the trash can:
and a hinge pin doorstop spacer:
