The short answer is 'all of them'. According to emedicine.com, both divisions
of ultraviolet light that we see at ground level, (UVA and UVB) can cause
damage to skin tissue.
UV-B is 85% stronger in terms of its effects but UVA
is much more prevalent at ground level, so both types are unwanted.
The only way to protect yourself is to cover up and stay out of the sun. With the added problem of holes in the ozone layer the importance of blocking UV from damaging your skin is higher than ever.
Using Outdoor suntan products such as hats and parasols and of course sunblock creams will protect you from the serious damage posed by excessive exposure to ultraviolet rays.