What is the toughest job in the United States Coast Guard? Why?

Every job! Each works tirelessly for the mission!

Buoy Tenders can have extremely long days of physical labor. They're also my favorites and have been the majority of my 10 year career.

i served in the '80 s onboard WMEC Dependable (626) she proudly displayed a golden pot leaf on her mast. = 1 Mil Tons seized. In addition to drug interdiction patrols we did immigration control, sar, and constant training.."Always Ready", all this with a $1 M bounty to who ever could sink us. She is still on duty.

As in any other branch of the service...

....Spouse.

Search and Rescue Swimmer?

I crossed deck and served nearly 10 years in the Coast Guard Reserve. Between the SAR swimmers, the Law Enforcement Dets, and the old Maritime Safety Office (MSO), the MSO types have the toughest job. They're responsible for vessel inspections, keeping the ports and harbors clean, and generally enforcing the national and international shipping operations standards.

I have a two-part answer.
Being a Marine Inspector. There are so many rules, regulations and procedures within USC series.
And working within the National Strike Force. That is a very challenging mission.

Cleaning the mud off the bottom of their boats. :)

If you ever found yourself in maritime peril and required the services of the Coast Guard I suspect you might just have a change of heart. With 25 years in the CG behind me Buoy Ops on the black hulls (buoy tenders) Pulling buoys off station on to the vessel, scraping and painting them and them resetting on station with GPS pin point accuracy.

I guess it depends on the circumstances. I'd say overall, rescue swimmers (AST) have the most physically demanding job. There's also surfmen, who are a rare breed, but certainly have gained a reputation for taking the boats out in absolutely insane conditions for SAR cases. HITRON for LEDET trains to shoot out the engine block of go-fasts (drug boats) from a helicopter.