Here are the filters that we use frequently:
B+W UV 010 to take away UV light and for lens protection. The B+W KR3 (81C) is one of our favorites by far.  It reduces the blue haze on your pictures especially when using Fuji reversal film at certain latitudes.  The filter also takes away bluish shadows and cloud reflexions on water in sunny conditions. I also like to use this filter during cold, rainy and cloudy conditions in arctic regions.

The B+W Circular Polarizer we use a lot as well.  When you make as many shots of river views as we do it just is an indispensable filter. The Circular Polarizer takes away not only the reflection of the water but also from windows and several other objects. In some ways it can let turn out pictures better by taking away the reflection only. Forrest green can turn out much better in straight sun light as well. We also use this filter to show clouds better in some sunny conditions. The filter also works great for snow shots and too bright conditions. Our favorite lens for this filter is the 50mm 1:1.4 USM. In the past and in my early years of experimental photography I made wonderful pictures by using a linear and circular filter at the same time. You can turn day into night and can amaze yourselves by the awesome effects  created by this trick.

The last filter that we use is the B+W 501 and 502, they are better known as a Graduated ND filter. GND or Graduated Neutral Density filters are an absolute requirement when you make a lot of landscapes pictures. The filters reduce bright sky without affecting the foreground or visa versa. They are especially indispensable when shooting a dark foreground with a bright sky in the back.

For more information about B+W filters just have a look at: www.scheidersoptics.com

Nowadays I have made my own filters in Photoshop as well