I always get Hoya SMC's or B&Ws off ebay, I've noticed reflection problems from tiffen/sunpak.
UV: to protect your front element
Circular Polarizer: Eliminates some glare, richens colors especially blue sky.
ND grads: Good for sunrise, sunset and any shot where part of the BG is darker then the rest.
ND regular: If you're shooting in very bright light and want to open up your aperture more, these are great. Indispensable on a video cams for shooting WBing during the day.
check out this link as it shows what they do:
http://tiffen.dtopinc.com/products.html?tablename=filters&filter_format=Scre w-In