It is advisable to see a doctor

it would be better for you to see a doctor.

it would be better

It would be much better if you took some medicine>
.)As a piece of advice*

The answer should depend on the dialect not the basic rules of the English Language ,so the answer should be, with respect to my friends the teacher of English, is one word like
"awful "

It's necessary to see the doctor.

it is better to see a doctor

it's advisable to see a doctor

It's very bad to hear that.

It is advisable to see adoctor

is necessary to see adoctor

it's important to see a doctor.

It's advisable to see a doctor

it is advisable to see aoctor