What is the biggest bill in the world?

It depends on the definition of "biggest". If you mean by denomination, the winners include a bill denominated at 100 trillion (10^14) Zimbabwean dollars, and a 1924 German banknote from the Weimar Republic that carried a similar number of Marks. However both of these bills were issued during times of hyperinflation so their actual purchasing power was quite small.

Today one of the bills with the highest purchasing power is the EU's €500 note, equivalent to about US$600 in mid-2015.

During the mid-1930s the US printed bills with a denomination of $100,000 but these weren't put into general circulation; they were only used for interbank transfers.