In order from best to worst:
Trolls (It's fun to mess with them as you not only get a crapload of points from arguing, but you get to be creative, get lots of free laughs, and it allows you to sharpen your witty comeback skills.
2. Idiots, because it's fun just to sit back with popcorn and watch how hard they fail.
3. Porn spammers. Sometimes you can get into some arguments with them, but several points off due to *retches* ...that.
4. Illtrueminatis: Most of the time you can't even get anything out of them except "Bush did 911" and "blah blah blah illuminati confirmed". I'd be more entertaining watching paint dry.
5. Downvote fairies. The bane of all imgflippers.