1番の理由って英語でなんて言うの？

There are many ways to explain that the reason you are giving is the first one you can say:

"The first reason is..."
"The number one reason is..."
"The main reason is..."

"The number one reason", "the main reason", and "the most important reason" all label a reason as having the highest importance amongst all other reasons.

Examples:

A police officer may say to a car driver "The main reason I pulled you over today is because you were driving too fast, but I also noticed your brake lights are not working.". We can see there were 2 reasons why the driver was stopped, but the police officer was more concerned by the driver speeding.

"My number one reason for moving to Australia was to practice my English.".

"The most important reason to do well in school is so that you can get a good job when you grow up.".

"The best reason" is what you can call the reason that is the most important out of all.

The three sentences you see provided above are excellent ways to express to your listener which reason is the most important. In these three sentences you will notice the adjectives primary, principal, and main. Although these words have different meanings, in these sentence they all mean first or key.

The number one reason is best described as the reason that is the most important out of a lot of other various reasons.

If something is the 'most important' it comes before anything else - so it is more important than anything else. 'Number one' means it's at the top of the reasons
So you could say 'Most important reason', 'Number one reason' or 'The main reason' these would all mean it's the best reason to give