Here is the current list of top 20 countries on IQ OPTION as on 2017:
Please note that in 2022 this Top20 looks different. The main reason is financial regulation. Especially in EU the government stopped brokers to promote some popular, previously non-regulated assets, like Binary Options.
The above list of 20 countries is more or less valid if you ignore EU countries. Also, UK is probably not on that list anymore. South American countries like IQ Option in Brazil would probably jump in the ranking.