Fair skinned: burn rather quickly without sunscreen. SPF 30 should provide sufficient coverage for any skin type, but is ideal for a fair skinned person who does not want to tan or burn.
Medium skinned: people are well suited to use SPF 15 or higher.
Dark skinned: people do need sunscreen to protect their skin. Dark skinned people would benefit from SPF 8, unless they are in the sun for long periods of time, in which case SPF 15 or higher would be advised.