I think, if I was pressed to say what heresy I hate most, it would have to be the worst of the "Word of Faith" movement, also known as "blab it and grab it" or "name it and claim it."
While this movement seems so friendly at first, with everyone telling you that you need only ask for what you need/want, it can get very nasty. For instance: if faith is a force like electricity(taught by Kenneth Copeland, among others) that you can use, then why are some prayers not answered? Because the asker didn't have enough faith, or the right kind of faith, or the right words to unlock the faith...and so on and so forth.
This in the end eliminates trust in God to know what he's doing. The believer ends up trusting only himself, and driving himself into madness when life simply does not go his way.
This is profoundly evil, and incredibly tempting. After all, isn't it nice to believe that God wants to give you only what you want? But this is evil, divisive, and deeply unbiblical.
In this view, how do you view the sick in your church? Oh, they must not have enough faith: I'm healthy, so God likes me better. Well, tell that to Joni Earickson Tada. How do you view the poor or homeless, on this view? Oh, they must not have faith enough for what they need. I don't need to help them, they need to help themselves.
If I had to pick my most hated heresy, this would be it.