The main thing is the most important one.
...one of the main tourist areas of the city.
You use major to describe something that is more important, serious, or significant than other things.
Lack of drinking water was a major problem.
Principal means first in order of importance.
Salt is the principal source of sodium in our diets.
A cardinal rule or quality is the one that is considered to be the most important.
As a salesman, your cardinal rule is to do everything you can to satisfy a customer.
The foremost thing or person in a group is the most important or best.
He was one of the world's foremost experts on the subject.
A leading person or thing in a particular area is the most important or successful one.
...a leading member of the city's Sikh community.