I have no grammar rule that I recall, but I hit the Google/One Look  and did some reading and here goes my personal opinion (take it as a hunch, then.  An educated guess...)

   http://www.onelook.com/?w=best*world&ls=a

1. best restaurant in the world
2. best skylines of the world
3. best tennis player in the world
4. best thief in the world

    From this piece of text, I gather that if you have ''more than one best'' a set of "best things" around, then you use OF.    For example: Brazil has the best soccer players of the world. (sorry, I couldn´t help bragging about some!)

     On the other hand, if you have a thing that is ''one of a kind'' (unique, being the only one) it will be ''in the world".

Language activities for teenagers - Resultado da Pesquisa de livros do Google
Seth Lindstromberg - 2004 - Language Arts & Disciplines - 225 páginas
... countries (eg Italy→Italian) and the grammar of plural countable nouns (eg ... best in the world! (Advise students that the best of the world is wrong. ...
books.google.com.br/books?isbn=052154193X...

            Here the grammar of it, has to do with plural countable nouns, technically, I mean. Someday I will master the grammar lingo and explain in its terms. But no right now.  It´s open, to more advanced contributors, then.

http://www.bbc.co.uk/worldservice/learningenglish/communicate/blog/student/archive/2008/04.shtml
"I tried to reword the following sentences:

1) This avenue is related to the Tango.
2) Palermo Woods: They have 80 hectares
3) This street is full of paintings and sculptures
4) It’s one of the fewest squares that are totally asphalted
5) It’s among the best in the world "

    Here in the BBC´s blog, the suggested correction to a learner, but maybe it is a Brit. English thing.

www.islandreefjob.com/ - Em cache

Island Caretaker Blog
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Island Caretaker Blog - The Best Job in the World.

A unique, one of a kind.

www.nacubo.org/documents/business_topics/AssessingImpact.pdf

   Looks like the American people use best in the world as well, not a single OF THE WORLD in this document pdf.

http://en.wikipedia.org/wiki/Restaurant_(magazine)_Top_50
"...a British magazine which produces an annual list of 50 restaurants ranked to be among the best in the world based on a poll of international chefs, restaurateurs, gourmands and critics. The list is published in April of each year (26 April in 2010)"

         Once more, in the world.

The Best of the World's Classics, Restricted to Prose, Vol. IX (of ...
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1 May 2009 ... Download the free ebook: The Best of the World's Classics, Restricted to Prose, Vol. IX (of X) - America.
www.gutenberg.org/etext/28653 -

                      The best of the World´s classics, a group of classics then.  Albeit the best ones.

Barack Obama shirtless: Best of the world leaders? - Telegraph
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23 Dec 2008 ... Barack Obama's impressive physique is the talk of the web, after he became the latest world leader to be photographed without a shirt.
www.telegraph.co.uk/.../Barack-Obama-shirtless-Best-of-the-world-leaders.html -

         Same as before.

     Well, I am explaining about what seemed the correct way [grammatically, albeit I am playing by ear, by instinct...seeking a pattern here]

     However, many native speakers will often use both ways (with both prepositions) to convey the message of "being unique/being one of a kind/the only one in the world...".   

     If so, mainly if it is in the middle of a conversation, why spoil the party? ignore your high expectations and the necessity of being right and enjoy the company, why not?

    Hope it helps.