Which folders get migrated to an SSD ?

I have a single HDD on my Windows 7 PC. I realize it will be impossible to migrate everything over to a smaller SSD. That I’ll have to backup folders such as documents, movies and music to another drive & then delete them from the HDD in order to fit on the SSD. But other than all the stuff in the Users folder, what else should be off-loaded to another drive? And what should be migrated? My...