Equally, you should never use "Dear Agent." These openings make the recipient of your query feel like they've been the recipient of a mass-email, and nobody likes that feeling. (I appreciate the irony of an editor telling us to query widely, and then saying that they don't want to feel like they're just one of a bunch of people receiving your query, but still. It's just not nice to be MADE to feel that way, even if it's a little bit true.)
You should also not open with anything like "Dear Agent Awesome," or "Dear One Who Holds My Dreams in Her Hands," or "O Worthy One," or anything like that. Some people think it makes you look witty, but it doesn't. It makes you look like a fruitcake, and we all know what happens to fruitcake: it ends up in sitting in a tupperware for 12 months until it finally sprouts a mould spore and you have an excuse to throw it in the trash. You don't want that to be your manuscript mouldering in some editor's trash.
Also, the fact is that most of the people who get your query are going to have to pass on it, for perfectly good reasons, which is fine - you only need one person to say "yes" - and saying "no" to someone is HARD. It sucks more than my mother's Dyson. Nobody likes turning people down. So don't make it harder for them by actually stating in your opening how high your hopes are that they will like your work.
Keep it professional. Use his or her last name. And that's it for queries for today!
For more #AtoZchallenge posts, check out the list here.