Filters with only mild effects are rather pointless on small-sensor cameras, as filters are very thick relative to the focal length, and have more adverse impact on the optics than on 35mm or large formats. UV or clear protective filters will mess with the lens quality for very little benefit.
I stick to polarizer, neutral density and IR. In my case, KSM C-POL MRC, ND 0.6 MRC, 092 IR - all B+W, which are still somewhat reasonably priced and obtainable in Germany. Elsewhere, you pay quite a "Leica premium" on them and might want to go with another brand.
Owning and using mostly film cameras for almost three decades, I have many other filters and got me adapters to fit them - but besides red, orange and yellow for some sub-IR black-and-white work, none of the old film filters proved useful. Among recently fashionable filters, the only ones that I own are:
486 UV IR CUT - no idea why that is so much in request that that many shops stock it, as the effect usually is less than impressive. In my case, it is used on one of my video cameras where I had the fixed internal IR blocking filter removed for IR use, and hence needed an external replacement. About once or twice a year, it comes in useful on other cameras, to fix IR related foliage colour problems on some odd plant in odd lighting, but that is rather out of proportion to its very high price. And due to its working principle, it has a growing amount of blue vignetting at angles beyond 35mm(equiv), which limits its usefulness on wide angle cameras like the GRD/GXes.
491 Redhancer - a filter with a narrow band-stop somewhere in the bright green range, which will often improve the brown-green separation. Its (IMHO ugly) colour cast has been abused a bit too often in recent landscape photography, but that can be removed in postprocessing without affecting the improved separation. Again, not cheap, but might be worth consideration if you do lots of plant and landscape images.