Sysadmins can be found joining sys.server_principals with sys.server_role_members.

DB_owners can be found joining sys.database_principals with sys.database_role_members.

The other permissions that allow to drop a database are CONTROL at the database level and ALTER ANY DATABASE at the server level:

SELECT pr.name

FROM sys.database_permissions AS dbp

INNER JOIN sys.database_principals AS pr

ON dbp.grantee_principal_id = pr.principal_id

WHERE permission_name = 'CONTROL'

SELECT pr.name

FROM sys.server_permissions AS dbp

INNER JOIN sys.server_principals AS pr

ON dbp.grantee_principal_id = pr.principal_id

WHERE permission_name = 'ALTER ANY DATABASE'

Hope this helps