It depends. Some people like manual labour--they like the physicality and challenge of it and the freedom of having no homework after finishing for the day. Some people like desk jobs and using their mind instead of their body. It depends really.
Anyway, for an 18 year without a degree, I would the hardest physical job would be that of a soldier in the infantry. The hardest mental job would probably be a boss for a large business (I've heard that most people who have succeeded in business don't have a uni degree). Dunno, my two cents.