Sugar Maples have the highest sugar content. You can make syrup out of other trees, including birches, but the sugar content is so low you need much more sap, or they just don't produce much sap. This is a great study:
https://employees.csbsju.edu/ssaupe/CV/conger_final_report.pdf
https://employees.csbsju.edu/ssaupe/CV/conger_final_report.pdf