Based on a comparison of 154 countries in 2016, Oman ranked the highest in military expenditure as a share of GDP with 13.7% followed by South Sudan and Saudi Arabia. On the other end of the scale was Somalia with < 0.001%, Equatorial Guinea with 0.179% and Mauritius with 0.188%.
Average military expenditure as a share of GDP reached 2.22% in 2016 in the World according to the SIPRI Military Expenditure Database. This is 1.83 % less than in the previous year and -5.64 % less than 10 years ago.
Historically, average military expenditure as a share of GDP reached an all time high of 6.01% in 1960 and an all time low of 2.18% in 2000. The average annual growth amounted to -1.76 % since 1960.
Data for more countries are available at Military Expenditure (As % of GDP) indicator page.