B+W, Heliopan and Hoya are all good, just choose the amount of darkness you want to add. I generally load HP5+ or Tri-x and carry a B+W 0.9 filter for three stops effectively bringing EI 400 down to EI 50, so that's f16 @ 1/250th reduced to f5.6 @ 1/250th in bright sunshine. Of course you can also keep the lens stopped down and use slower shutter speeds. It just adds flexibility and saves carrying more than one film stock and/or body.
Mark