There are a few filter options to consider depending on your needs:

1. UV Filter:

   • Primarily used for lens protection against scratches, dust, and minor impacts
   • Has minimal effect on image quality in most scenarios
   • Good to keep on the lens at all times for peace of mind
   • Recommendations: B+W XS-Pro UV MRC-Nano, Hoya HD3 UV, or Tiffen UV Protector

2. Circular Polarizer (CPL):

   • Reduces reflections and glare from non-metallic surfaces like water, glass, and foliage
   • Enhances color saturation and contrast, especially in blue skies
   • Useful for landscape and outdoor photography
   • Recommendations: B+W XS-Pro Kaesemann Circular Polarizer MRC Nano, Hoya HD3 Circular Polarizer, or Breakthrough Photography X4 CPL

3. Neutral Density (ND) Filters:

   • Reduces the amount of light entering the lens without affecting color
   • Allows slower shutter speeds or wider apertures in bright conditions
   • Useful for long exposure effects like blurring water or clouds
   • ND filters come in various strengths (ND2, ND4, ND8, etc.)
   • For a versatile option, consider a variable ND filter that covers a range of stops
   • Recommendations: Tiffen Variable ND, B+W XS-Pro Digital ND Vario MRC-Nano, or PolarPro Peter McKinnon Variable ND

In general, stick with reputable filter brands like B+W, Hoya, Tiffen, Breakthrough Photography, or PolarPro. They offer high-quality optical glass and coatings to minimize image quality loss.

I'd suggest starting with a basic UV filter for protection, and then adding a CPL or ND filter as needed for your specific photography interests. Remember to avoid stacking too many filters to prevent vignetting and reduced image quality.