UV (depending on how many filter stages you have, you could get a screw-on/lens-mount UV)
ND .6
ND .9
Grad ND (soft edge, at least .6 and .9)
Circular Polarizer
That should get you started. Any of the color effect filters are almost obsolete, since those can be created in post with ease. But whatever you do, don't cheap out on the filters. Go for something like Formatt or Schneider.
Tiffens are mid-line, I have a couple of NDs but for grads and polas, I use the Formatts, they are excellent. Some DPs and Colorists feel that the basic stock glass that Tiffen uses lends a greenish cast to the picture.