The letter 'E' is my favorite, because it is the most common, therefore the easiest to construct words with. This comes especially handy when playing
Bookworm (danger: don't click if you have work to finish by a deadline)
Seriously, 'E' is one excellent letter. Not only is it the most used, but it scores you points in Bookworm (and, therefore, chicks)! What more could you want?