DSLRs need few filters as most effects can be better achieved in Photoshop.

For example, red, green or orange filters used to increase contrast in b&w photography are obsolete, as you have far more precise control over the contrast in Photoshop.

Similarly, there is no role for warming filters when white balance can be adjusted both in-camera and when opening the RAW file.

But a neutral density (ND) filter still has a role to play when you want to use a slower shutter speed for artistic purposes, for example photographing a waterfall or fountain.

And I do believe in using UV filters to protect lenses against scratching. Yes, most of the protection will come from the hood (and I always use those), but for any expensive lens with an exposed front element, I also keep a UV filter permanently in place.

If you're going to do this, however, you need to use high-quality filters. You wouldn't spend several thousand pounds on a top hifi system and then listen to it through a pair of cheap headphones you picked up on a market stall. It makes just as little sense to spend a fortune on high-quality lenses and then stick a cheap filter in front of them.

Cheap filters are made from window glass. The thickness will vary, and they will have other imperfections that will negate the whole point of a high-quality lens.

Proper filters are made from optical glass, with the same quality control procedures applied to lens manufacture.

From the tests I've seen, and from recommendations, the hierarchy looks like this:

Best:
B+W and Heliopan. Both use German Schott optical glass, and both use brass rings. Brass doesn't contract & expand with temperature to anything like the extent aluminium does, so there is far less chance of it binding on the lens or coming loose. I haven't seen test data or info on Leica filters, but I would be pretty confident these would fall into the same category.

Next:
Nikon and Hoya. Both use optical glass, though not Schott glass as far as I know.

Brands to avoid:
Tiffen, Cokin, Singh-Ray and Jessops. These are made with either window glass or plastic. Enough said.