The simple answer, is none of the following. Andrew Lownie, of the Andrew Lownie Literary Agency, has come up with a list of some of the ways he has been addressed by would-be clients....
Dear Respectful One
Dear Potential Partner
Dear Sir Andrew
Dear Mr or Mrs Agent
To whom it may concern
(You can find the complete list at his website). I'm seriously hoping you're laughing at this rather than blushing guiltily, but how should it have been done?
Dear Andrew Lownie
That's it. It's correctly spelt, and there isn't a title. This is particularly important when writing to women because of the whole Mrs, Miss, Ms business. A Mrs may be deeply offended at being called a Ms. A Miss may prefer the anonymity of Ms. They may be married, but use their maiden name at work, or vice versa. I was once told rather huffily by one editor that she was married so she certainly wasn't a Ms or a Miss, but she chose to use her maiden surname professionally so she wasn't Mrs maiden name either.
Another problem with titles is playing guess which sex as there are several names that can be unisex, like Val, Nick, Jo. And some names can catch you out. Leslie for men, Lesley for women you might think. But my middle name is Leslie, because I'm named for my grandfather and my father didn't know there was a female version. I've met another female Leslie in publishing, spelt that way for exactly the same reason.
So, no title is the best policy. And check, and re-check, the spelling. Is it Carol, or Carole? Katherine, or Catharine? Get the name wrong and your submission goes straight onto the 'no' pile.
Perhaps all this sounds too fussy. Perhaps you feel that agents are putting themselves on a pedestal way up above everybody else. In which case, why not just stick to my favourite from Andrew Lownie's list: Dear God's Elect. That should do the trick.