If you could only have one test...which would it be?

[Totoro]

Since heart disease is our #1 killer, I'm surprised no one said a coronary angiogram.

If the test were free and could be almost guaranteed of no further complications, I'd love to know what the inside of my coronary arteries look like.

Statistically speaking:
List of preventable causes of death - Wikipedia, the free encyclopedia

So, a colorectal cancer test. Alternatively, a medical competency test for the doctor treating me (does that exist)?

A coronary angiogram indeed, once I'm 50 years old. Before that a generic cancer test (early stage - also doesn't exist?).