Dear Maysam, I am doing my PhD program in one of the best European universities, it is Budapest University of Technology and Economics (Hungary). if you can get an opportunity from Stipendium hungaricum scholarship this will be a good choice and off course this depend on your skills and the cooperation program between your country and Hungarian government. Regards
This decision would depends on thinks as: language of your knowledge, field of study, your background, etc. The finantial support in Europe and US is interesting. Duke University in the US or both the University of Valencia and the University Politecnica of Catalonia in Spain would be good options.
I think your research back ground is about nanofluid. Hence you can apply Australia, South Africa, China and India. When you search well you can find professors.
In past few years, China has excelled very well in higher education. Some of the China's universities are in the top lists. Even if you see on research gate, large number of papers are from China. Even some universities in China have instructional language as English which can be attractive for international students. However, there are other aspects besides academic issues. For example, a large number of graduate students do migrate every year after graduation to the countries of their study. In that case, one may select a country for study where they want to settle ultimately. After doing a PhD from a University, most countries including USA provide some options for job. These jobs may ultimately lead to the migration of that family. It is generally a win-win situation because the country gets a well educated and highly qualified active brain while the family settles where they want to. I do not know if such option is available in China or not however I know such options are available in USA, UK, Germany, Australia, new Zealand, Sweden and Canada which is one of the reason for their popularity in higher education destinations.