When it comes to C-Pols, I recommend the B+W Kaesemann in particular.... They are the top-of-the-line, give smoothest effects and are very neutral in color. Very well made.
I'm not a big fan of step up rings and using oversized filters, simply because it usually precludes using a lens hood. When using a filter you have two or more additional glass/air transitions to be concerned about, so a lens hood is even more important than when shooting without any filter. I carry at least one C-Pol filter sized to fit each lens (58, 72 and 77 in my case... plus 52mm drop-ins). I do use step rings, but only to stack all my filters for convenient, safe storage.
You might be able to prioritize using on one lens or another, getting only the most important size right now.... for example, among your lenses I'd be much more likely to use a C-Pol on 24mm and 24-70, both of which use 77mm. Of course, you might feel differently.
You are looking at ND filters? For what purpose? I ask because some folks mix them up with Graduated ND filters. On the other hand, particularly with folks now using many of these lenses and cameras for video, regular NDs are becoming a lot more popular.
Graduated ND are better bought in the rectangular type. Screw in type place the transition at the middle of the image, every image you take. Rectangular are used in holders that allow you to adjust and position the transition to match the horizon, for example, in your images. For your lenses, you'd need "Cokin P" size at a minimum. Not necessarily Cokin filters (which are cheap and a good way to experiment, but might be hard to find since Cokin is in bankruptcy, last I heard). Other manufacturers such as Lee and Singh-Ray make rectangular ND-Grads in "Cokin P" size. They also sell holders and accessories to fit the same. This type filter can be bought in optical plastic (some are quite good) and glass, but most are uncoated. A hood of some sort is a good idea.
With ND and ND-Grads you'll have to decide on the strength you need too.
ND-Grads for use with scenic shots, I'd recommend 1-stop and 2-stop, which can be combined to give 3-stops if ever needed. With ND-Grads, you also can choose between hard and soft transitions. It's difficult to recommend which (many folks end up getting both).
There are variable NDs, or a wide variety of them in different strengths. They are relatively expensive. As far as I know, the more affordable variable filters (similar to using a C-Pol) are not multi-coated or particularly high quality glass. The best variables (perhaps Singh Ray?) are quite expensive... probably multi-coated and good glass.
Non-variable ND filters come in a huge range of densitities. It completely depends upon what you want to shoot and the ambient conditions, how strong you'll need. ND filters are widely available from different manufacturers, including B+W who offer both multi-coated (MRC) and less expensive single or uncoated versions. Therre are other good brands, besides B+W: Hoya (HMC or SHMC "Pro 1") and Heliopan (SH-PMC) I can vouch for personally. I also hear a lot of good things about Marumi brand, but haven't used them. Even Kenko, Tiffen and some others make some high quality versions. My main concern with a filter of this type would be mulit-coating and completely neutral color.
I have not used Calumet's filters. They are undoubtedly made for them by someone else (pretty much everything under the Calumet label is outsourced). They might be pretty good. Schneider Kreuznach is a quite good glass and lens manufacturer. The best filters from the German manufacturers use Schott glass. That's all Heliopan uses... I think they get their glass from Zeiss. But some of the Japanese manufacturers pretty much match it for quality. In fact, many Zeiss, Hasselblad and other highly regarded lenses are now being made in Japan.
