If you can only afford one, I'd definitely recommend a circular polarizer (See list below). Down the road, look into a 3-and/or 6- stop neutral density filters. These three take care of most lighting situations.
B+W 77mm Kaesemann Circular Polarizer MRC Filter
B+W 77mm 0.9 ND MRC 103M Filter (3-stop ND)
B+W 77mm 1.8 ND MRC 106M Filter (6-stop ND)
Step up rings to fit your other lenses, depending on your lens collection:
B+W Step-Up Ring (Lens to Filter) 72mm - 77mm
MRC stands for Multi-Resistant Coating and costs more. The regular versions work just fine. Look into getting the slim threads to avoid vignetting.
You can use these individually or configure them differently by stacking them, although I normally would only stack one CPL with one ND filter at a time to avoid vignetting and image degradation. Asides from the obvious use in landscape photography, the polarizer and ND filters also help to keep the lens aperture wide, ie. shooting wide open at f/1.2, f/1.4, etc. in harsh lighting, ie during bright sunny days. I would not use them at night.