Because it's adorable:
Luna and Neville. I know they both have canon husbands and wives, but JKR didn't tell us much about either of those names in the books. I gotta do what I can to imagine Neville with a girl who's worthy of him.
Because it's hot:
Basically any m/m/f configuration. I'm sorry, I've thought a lot about it and I just can't choose. I should create a reading list for y'all. Stories about one woman with two men as her playthings, mmph.Sorry, where was I?
Because it's plausable:
Draco/Hermione. Sure, he's a little ferret and she's a little goody-goody, but I love stories where they get over all that, preferably after some grudge sex, and then set out to get things accomplished. Solve mysteries, battle Inferii, that sort of thing.
Because they're alike:
Remus/Severus. This is my slash otp. I especially adore stories set between book 3 and book 6, when they're both spies under Dumbledore's thumb, knocking around Grimmauld Place, or meeting to exchange information vital to the Order. Except please, no situation involving mpreg, and no alpha-omega stuff. Am I weirdly specific or what? Moving on ...
Because they're opposites:
Severus/Luna. Usually the Severus character is fairly OOC for it to work, but once in a while, someone gets the chemistry right. Which isn't to say I don't love them all, because I do. Mercy!
Because I like it in canon:
Lucius/Narcissa. I don't trust Lucius at all. He's a loose cannon. So I almost always like to see him safe in the capable arms of his wife.
Because of fanwork:
Severus/Hermione. (Were you beginning to wonder if they would ever get mentioned?) I'll be honest, I got interested in HP fanfiction because of my OTP. But one of the first authors who ever moved me with her writing, teddyradiator, took me in and showed me the incredible beauty of this pairing. I particularly love how vigorous the HGSS community continues to be.
Because it hurts so good:
Remus/Tonks. Remus is mine! Mine, I say, mine! No jumped-up little blue-haired floozyis going to take him away from me! Except Tonks is awesome and cool and I want to be her.
Because of subtext:
Lucius/Ginny. There's a world of hatred, grudges, slow-burning maliciousness (Lucius), and hot-tempered fury (Ginny) that must always rear up when those two get together.
Because they're my OTP:
Remus/Hermione. I just love the thought of them together. What they lack in exciting emotional fireworks, they always make up for in heartfelt passion and depth of true feeling. Since joining HP fandom years ago, I've met a very small but loyal cadre of Remus/Hermione shippers. We're like a little secret society, tucked safely under the huge shadow of HGSS. And whenever a writer offers to write a one-shot about someone's OTP, I always put in for these two. I'm trying in my own small way to keep the ship afloat.
Because I CAN!
Weasleycest. Let's leave it there.