Paranormal romance / urban fantasy. Vampires and werewolves and zombies.
Zombies are done, vampires are way overdone if you are trying to break in, and werewolves are a tough sell (because again, done).
The hot stuff in paranormal romance at this VERY moment is fallen angels. But that's if you have a completed manuscript to shop around in the next few weeks, because it's pretty much saturated now that every house has bought up a ms or two. I'm already hearing word that a lot of people have angel manuscripts and are hearing "Sorry, already got one."
Here's the thing with the trends (at least in romance). One angel book makes a splash and then every house thinks, "OMG we need something like that on our list." Everyone -- editors, contracted authors looking for a new project, agents, etc -- scramble to get an angel book in front of editors. There's a mad rush and flurry for a few weeks, an angel book or two get acquired, and then every aspiring author starts paying attention because, wow, angels are EVERYWHERE on Publisher's Marketplace, so they must be hot.
Except by the time you decide to write an angel book? Everyone already *has* one. And now they're going to wait and see how they do before wanting more. If every angel book that comes out in the next 2 years tanks like a mad tanking thing, angels will be done. If every angel book sells through the roof, the publishers are going to sit back and consider more projects -- as long as they don't compete with the existing angel projects, mind you -- and look at acquiring more.
So when you get wind of a genre starting to become 'hot', it's already winding down. If you have an agent, a good track record of sales in your genre, and you can whip together an amazing proposal in the space of about a week, you might be able to jump on board. Otherwise? You just need to hope the particular trend has major, major legs (like vampires).
Case in point --
Pride and Prejudice and Zombies. Did amazingly well. Everyone scrambled to have the next book. Some publisher paid a ridiculous amount of money for
Queen Victoria: Demon Hunter. It came out this last week. Did you hear a peep about it? No, me either. That trend is already dead in the water.