Best filter for dslr cameras?

Really broad question... what kind of filter? What are you going to use it for?

The most commonly used type of filter on a dslr is a polarizing filter. This is the equivalent of wearing sunglasses. Clouds are whiter, skies are bluer, and the whole image is darker.

Many people recommend using a clear filter to protect the lens. This isn't great advice and NO pros do this because it reduces sharpness and contrast, and also makes lens flares more common.