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The most controversial—and effective—ads ever?

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Advertising has long been recognized – and criticized -- as an efficient way to persuade us to spend our money. But sometimes the "Mad Men" have chosen to take a risk, go beyond the "soft sell", and seek to challenge the way we think.

They have flirted with race, religion, sexual orientation and more to make people question their belief systems – and boost their brand at the same time.

"The cleverest thing an advertiser can do is the opposite of what everyone else is doing," said Daniel Hennessy, U.K. chief creative officer at Geometry Global agency, who compiled this list of controversial ads for CNBC. "Sometimes it's risky, yes; but it's better to be talked about than not talked about."

Click ahead to see some ads from around the world which aimed to change the way that people think.

- By CNBC's Katrina Bishop @KatrinaBishop

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