I was taught that "To Whom it may concern" is outdated and one should always use "Dear Sir". If Dear Sir - use "Yours faithfully" - easy way to remember - never use the two "S's" together (ie Sir and sincerely).
Definitely Yours faithfully, however I have noticed a new phrase on the block - "With kind regards" is becoming very popular where I work. Or have I just seen that on a formal work email?! Maybe it is just for emails.