To Whom It May Concern,
I always think this sounds overly formal and somewhat snotty, for all that it's probably the one I most commonly use. It also always reminds me of a Peanut's cartoon: "To Whom It May Concern: Dear Whom,". Besides, how do I know that the person reading it is going to be concerned? I figure the letter is going through a lot of hands before anyone gets even remotely concerned about it.
Dear Hiring Manager,
"Dear" is such a familiar term these days that it seems incongruous to me to follow it up with an impersonal title like "Hiring Manager". Particularly since the chances are that a lot of people are going to read the letter other than the almighty Hiring Manager, and I don't want to lose their interest before I've even caught it just because the letter salutation gives the impression that I'm talking to someone else.
So informal. It's generic enough now, but it almost seems flippant. In this day and age of mailing in for jobs, I suppose it's okay. But still, it does seem a little light-hearted.
Anyone have any other suggestions?
(Of course, if the name of the hiring manager or their exact title is known, it's best to actually use those. I'm just wondering about all the other cases.)