Who is the tallest guy in world?

Yao ming.

The tallest living person not recognized by Guinness is Leonid Stadnyk of the Ukraine, who turned 40 in 2011 and whose height is 8' 5.5" (257 cm). He refuses to be measured by the new standards that Guinness adopted in 2000.

According to Guinness, Sultan Kösen of Turkey, who turned 30 in 2012 and whose height is 8' 3" (251 cm), has been the tallest living person since September 17, 2009.

(The tallest person ever measured by Guinness was Robert Wadlow (1918-1940) of the United States. His height was 8' 11" (272 cm).)

(FYI: Yao Ming is the 31st-tallest living person and the 4th-tallest living person in China.)