Bramer, W. M., Rethlefsen, M. L., Kleijnen, J., & Franco, O. H. (2017). Optimal database combinations for literature searches in systematic reviews: a prospective exploratory study. Systematic reviews, 6(1), 245. doi:10.1186/s13643-017-0644-y
These are commonly used databases for systematic reviews. Depending on your research question, you might want to consider other databases.
ASSIA indexes and abstracts articles on health, social services, psychology, sociology, economics, politics, race relations and education. Sources are from 16 countries, including the UK and US.
A collection of systematic reviews on the impact of health-related interventions & treatments. Updated regularly.