I particularly draw your attention to Lynn's point about the pointlessness of saying "I chose your company because..." This habit has come about because of writers' genuine and well-intentioned efforts to prove that this is an individual query and the product of great research. However, think about it:
a) as Lynn says, it really doesn't matter why you chose themWhile searching for Lynn's link, I found this post of hers, containing more query letter noes. The woman is good. When she's not being bad.
b) it's a tad cocky, because there is a flavour of "how lucky you are that I chose YOU"
c) what if this is not the first agency you've tried? Or even the fourth? What you're hiding is, "I chose your agency because although you weren't top of my list of agents, you were at least on the list of agents who might in a million years consider my sort of book."
The lesson from both Janet and Lynn's posts is: put yourself in the shoes of the recipient and read between your own lines. Hear your own subtext.