Which essay topics are the most effective?

The best topic is one that brings your strengths or your uniqueness to life for the admissions committee. What that will be varies from person to person. The idea is to find some way to stand out—and if we could give you a simple formula for that, everyone would use it and it would no longer work!

Being unique doesn't necessarily mean using a cheap gimmick. An unusual approach will only work if there's substance behind it and it's relevant to your point. If done well, writing an essay in rhyme could help a future English major; the same approach carried out more sloppily by a math whiz would be silly. Your essay doesn't have to scream "I'm one-of-a-kind!!!" to be successful. On the other hand, be wary of clichés. Always dig for your specific, individual slant on a subject, and find some small way to show what makes you different from a similar student with identical grades and test scores.