depends on your web server setup tbh. generally something like 600, 640, or 644. This mainly depends if everything is runs under the same user (i.e. FTP, webserver etc), then 600 is best. Otherwise 640 is probably your best bet if all the different users are under the same group.
If you need write access to files (ideally you should avoid this where possible), then add 2 to those digits.