Granted, my kids are young, but I have two responses to "I hate you!" that seem to work so far.
The first is to say, "That's a punishable offense." Meaning, you'll lose a privilege if you talk to me this way. And I follow through. I tell them when things are calmer that "I hate you" is uncreative and lazy, and that they have a million different and better ways of expressing their frustrations with me.
The second is to say, "You know what? That really hurts my feelings." I don't say it with tears or a quiver in my voice, but more like a correction for a slip-up. More in the same tone as I'd say, "You know what? You left a mess all over the floor." I think it's an honest reaction but not the kind of reaction they were hoping for, (if they're even HOPING for a reaction - sometimes they say it because they're too frustrated to be articulate) and it tends to stop them.
Basically, I've adopted the stance that saying "I hate you" is beneath them and I'm hoping that they'll internalize this as they grow into the teen years.