Top domain rankings (e.g., Alexa) are commonly used in security research, such as to survey security features or vulnerabilities of “relevant” websites. Due to their central role in selecting a sample of sites to study, an inappropriate choice or use of such domain rankings can introduce unwanted biases into research results. We quantify various characteristics of three top domain lists that have not been reported before. For example, the weekend effect in Alexa and Umbrella causes these rankings to change their geographical diversity between the workweek and the weekend. Furthermore, up to 91% of ranked domains appear in alphabetically sorted clusters containing up to 87k domains of presumably equivalent popularity. We discuss the practical implications of these findings, and propose novel best practices regarding the use of top domain lists in the security community.