I want to pursue MS in Mechanical Engineering so which country I should choose from USA and Germany. And is GRE necessary for Germany.

Not all programs offered under Harvard SEAS require a GRE score. GRE is only required for applicants applying to the Master of Design Engineering or the MS/MBA: Engineering Sciences programs. 

The University of Florida admission requirements vary greatly by course. For some MS programs at the University of Florida, the GRE is required but for some, it is not. 

The GMAT may also be applicable for some programs. If your programme requires you to submit the GRE, a minimum score of 140 in the verbal GRE is needed. 

Other than this, the students are also required to meet the given admission requirements - 

• TOEFL iBT: 80 or IELTS for English proficiency 
• A UG Degree with a good GPA 

No, GRE is not mandatory for all universities. Top institutions like University of Toronto and UBC often require. Many universities like Simon Fraser University, Ryerson (TMU), and Carleton University make it optional or waive it if you have a high GPA and relevant work experience.

Candidates with a valid score of CAT/ GMAT/ GRE score are exempted from the written test. These candidates still need to complete the form by paying the application fees. However, these aspirants may get called for the IIM Trichy - Chennai campus PGPBM interview round. 

The University of Buffalo ask for GRE score for some PG programs. The university provides test option for international students. Students usually submit these score along with the transcripts, LOR, English language requirements to give a boost to their application.