Hello, 

There isn't any best stream as such for UPSC, it just needs a graduation degree with no set aggregate. People from different streams like humanities, science, medical, management have cracked this exam

Syllabus for prelims in combination with general studies are something every aspirant have to study. If among these subjects anything interests you, you can choose for optional. Otherwise, you can choose optional among the list provided by UPSC, as long as the subject interests you. People usually go for the subject they had studied at ug level, like medical, engineering, political science, geography etc. Choosing optional should be kept in mind in accordance to one's interests. Because there's so much to study, it's nothing less than study of graduation level. 

Thank you!