Buy your filters in the largest size you think you will need and use step down ring to fit them to smaller lenses... that way you will only have to buy the filter once. I have the 15-85mm too, but also a Sigma 10-20mm which takes a 77mm filter. I bought my polarizer and 10-stop ND filters in the 77mm size so they can be used on all of the lenses I own, including my 100mm macro and 55-250mm (both 58mm threads). Step down rings are cheap ($2 or $3), so it will save you a lot of money in the future if you're planning on buying larger lenses. The only disadvantage to doing it this way is that you may lose the use of your lens hood if you're using the step down ring. I have small fingers so I can fairly easily manipulate a CPL inside a lens hood, even with a 77-72mm step down ring (if you put the hood on first, you can then screw on both the step down ring and the CPL). If you have large fingers you may just have to ditch the lens hood or get a hood that screws directly to the filter. I hope this helps save you some money in the future
Also, I use the Marumi DHG Super CPL and I'm very happy with it. I've used B+W and Hoya filters as well, and they all seem like they are very good quality. My 10-stop ND filter is a Haida Pro II MC Slim which I've used a couple of times so far and it also seems like a good quality piece of kit, though 10 stops is kind of a specialised thing.