A klein bottle with wireframe style so the whole surface can be seen. With a solid surface the bottle looks not so exciting and the meaning is difficult to understand. So I made this version.
It prints without support, but isn't very easy to print, because it needs a precise calibration and good adhesion on the bed.
Edit: I tested it now with 0.3mm layers and it printed successfully (yellow image).
I printed it with 0.15mm layer, 0.5mm nozzle, no support and no infill. For better adhesion my printing bed is covered by Kapton tape. It took 2h and consumed 2m of my 3mm filament (15,5cm³).
(Same settings with 0.3mm layers took 50min and printed successful, too)
As there are bridges everywhere, you should calibrate your bridging in before.
Because the contact surface is very small, it's likely to move while printing, ruining the print. I've made a version with predefined brim, which can be more easily removed, because it doesn't fill the holes of the model. The model without brim is there too if you want to manage the brim on yourself.