What is the best HTML, CSS and JS framework for developing in your opinion? why?

Always use latest version now a days HTML5 and CSS3 is popular and About to JS.

You take any JS framework Like Angular JS, Node JS according to your application need.

Jquery is also a best option for DOM manipulation.

I am using Bootsrap, its good easy and flexible!

Basm allah alrahman alrahim

bootstrap , jquery

but to be a professional web developer , make your own framework ?

How ?

after a while of using bootstrap and jquery you will know every selector ( in bootstrap ) and every function ( in jquery ) , then you will be able to make your own requirments framworks .

Why ?

For semplacity ?

any web page have load time , if you want your website to be the best you must only use functions and class benefit your website pages , so you must develop your own [ the less codes line , the fast page load ] , and don't forget the less images ( and images sizes ) the fastest load that will make users never forget your website 

By the way wordpress cache plugin make your website very fast ( but you must use it after you can understand every thing about caching , cause it may convert your page to static page forever , if you use it wrong )

The answer is in your question as you said

What is the best HTML, CSS and JS framework for developing in your opinion?

In developing what? that is the part, If you figure it out, than you have answer. But it really boils down to experience, I have those questions but with time and different project I crossed over from bootstrap to Susy or some time I would stick to plain CSS without using any CSS frameworks.

Similarly while starting out with Angular Js, later on I realized that ember and react has much easier learning curve and documentation than the prior one.

It's best not to spend time comparing tools than to just get started what at hand. Later with trials and error one would realize when to choose one.

I hope this answers your question.

There's no difference to working with each one separate they all complementary

i think that the best is Bootstrap

then foundation

somme of my web application i have used semantic-ui 

there is pure.css by yahoo

and uikit witch is used by may wordpress themes