What is the best game in the world?

The 'best' game is just a matter of opinion, it depends on which genre of games you like, and how difficult it is to play. Assuming you are talking about Video Games, The best-sellers should be an indication of which games are most popular:

• Mario
• Pokémon
• Wii games
• Tetris
• The Sims
• Grand Theft Auto (18+)
• Call of Duty (Usually 18+, some 15)
• Final Fantasy
• FIFA
• Need for Speed

The above list have supposedly sold over 100 million copies throughout the franchise, with Mario selling the most at 262 million. However, this doesn't neccessarily mean the 'best' game is in the list.