Here's one that will blow your mind.
This is a ring with a central heliacal gear, and six additional heliacal gears:
I then merge the two parts together like this:
Now, when I print, this part will come off of the build plate and spin without any further steps needed, and without the gears falling out!
How? Because none of the pink parts actually touch the grey parts so they all print individually. Since they're heliacal, when the layers stack, the gears become "locked" into the surrounding ring and central gear. I'm printing one now and will post pics and maybe a vid of me taking it off the build plate so you can see (takes 3+ hours to print on normal quality).
Of all the things I've printed, these little gear toys are by far the most popular. They're mind-blowing and they act as fidget spinners so everyone loves to play with them and marvel at how they're even possible.