What is the best width of the web page in these days , because there are lot of screen with different sizes ?
The answer lies in your question...
Because people use websites with tons of different devices, from mobiles, to smart phones, to monstrous 50" monitor setups, it's best to use a liquid, elastic, or semi-elastic layout and not define your layout dimensions with a fixed width such as pixels.
that means , we have to do device specic web pages ?
No, it means the opposite; by designing your website as flexibly as possible. i.e. by creating fluid or elastic layouts and applying clean and modern coding techniques & technologies, you lessen the need for having to create different versions of your website to accomodate other devices.
The answer is: "the one that fits into my browser window".
Seriously, that's the answer. If it's at all possible (and 99% of the time it is), design a site that will stretch and squash to the available width. That way people with narrow screens won't be visited by the evil that is the horizontal scrollbar and people with wide screens won't have loads of wasted space down the sides.
You would be well advised to add a min-width (maybe 600px) and a max-width (maybe 1200px) to give some boundaries to the design, but within that, just go with the flow and give me the size that I want.