Which unit I should use in CSS when designing a web page

Question

I have designed and developed more than 10 sites, but I still have a doubt about the correct unit I should use. Should it be px, em or %

Answer 1

Use the unit you need in the specific context.

Unit   Description
====================
%   percentage
in  inch
cm  centimeter
mm  millimeter
em  1em is equal to the current font size. 2em means 2 times the size of the current font. E.g., if an element is displayed with a font of 12 pt, then '2em' is 24 pt. The 'em' is a very useful unit in CSS, since it can adapt automatically to the font that the reader uses
ex  one ex is the x-height of a font (x-height is usually about half the font-size)
pt  point (1 pt is the same as 1/72 inch)
pc  pica (1 pc is the same as 12 points)
px  pixels (a dot on the computer screen)

source: http://www.w3schools.com/css/css_units.asp