What is the best textbook (or book) for studying Etale cohomology?

Not a textbook, but a free PDF by J.S. Milne, http://www.jmilne.org/math/CourseNotes/LEC.pdf, pretty good IMHO. 


I'll complement the list of well known books on the subject by some freely available documents, which I find userfriendly. Here are great lecture notes , from a course that de Jong (of Stacks Project fame) gave in 2009. Edgar José Martins Dias Costa's short dissertation on the subject . Evan Jenkins's notes of a seminar on étale cohomology (click on the pdf icons). The arXiv notes of a minicourse given by a fine expositor, Antoine Ducros, which also cover analytical aspects of étale cohomology (used for Berkovich spaces). And finally a historic survey (in French unfortunately) on the genesis and successes of étale cohomology. 


Lei Fu, Étale Cohomology Theory is also nice and has not been mentioned yet. 


My first exposure to étale cohomology was through Bjorn Poonen's notes Rational Points on Varieties, Ch. 6. Not all of the big theorems are mentioned there, but it provides a great introduction to those who have had no previous dealings with the subject. 


I would highly recommend these notes by Donu Arapura for a good overview of etale cohomology, as well as this short paper by Tom Sutherland for an even quicker overview. 

