I’d say start simple and little by little build up to complexity. And complex doesn’t necessarily mean a huge robot, it can be a bottle of detergent, that thing has some funky ultra tricky shapes:

But if you want to go all the way, a car is always a mega challenge. Anything that has smooth curves yet sharp shapes is tough.
Mechanical pieces are always a go to as well:

The intricate shapes that flow into one another makes those things a headache.