I came a bit late to this discussion. I own exactly 48 filters (i have made a small excel file to know which are they and where they are), diameters 27mm (Zeiss Contessa) to 77mm.
I have filters of many brands and the worst filters can be easily singled out because they have some abberations or weird artifacts near the rim of the glass.
Then, you can easily check if the coating is good by getting a bright lamp and looking through the filter. The worst filters will get significantly lowered contrast. Same check can be used by looking through your viewfinder, with a lens on, and examining if the image drops in contrast when the filter is in front of the lens, when framing a bright object such as a desk lamp against a dark background.
The really good filters don't create a loss of contrast / added ghosts.
I usually don't like "protection" filters because all except the best filters will add a little contrast drop. And then many "protection" filters are not color-neutral. They also can create a loss of image quality, check out "Gary Reese Lens Tests" where he does some tests and the impact of quality can be strong.
I own filters of many brands: Hoya, Pentax, Nikon, Toshiba, Tiffen, BW, Kenko, Visico, Contax, Canon, Kaiser.
For me the best filters are the camera brand filters -- Pentax SMC, Nikon filters are great. Canon, i can't judge since the ones I have are on bad shape. I also have one Contax filter ('80s, red), also great. Nikon makes the best filter cases, btw.
Besides those, i find B+W filters really good, as well as most Hoya ones. Kaiser also seems good (german made, seem made by B+W). Kenko MC also seem good.
I usually avoid anything that says "vivitar" but i got a skylight filter marked "Vivitar Series 1 VMC" that is well coated and seems sufficiently well made, so it has the honor of sitting on top of my Nikkor-N 28/2.0, a nice lens.
I guess my most favorite filters are the bayonet Pentax SMC filters created for the Pentax 6x7 lenses. They are easy to attach and they are really, really well built. Those I do use as "protection" filters!