Ok. let me clear the air a little bit.
1. UV rays do not cause damage to your lens or hardware. Actually, UV is good as it kills fungus.
2. UV filter is more applicable in the film days, where film can be affected by too much UV light. Digital camera sensors have no problem with UV rays. People use UV filters more or less as a habit from the film days for protection over the lens. Nowadays, you see newer filters called "protector" and Nikon's NC (clear). These filters just protect and do not filter UV. The only thing you need to think about is multi-coating. Since adding a piece of glass in front of your lens can affect IQ in terms of flare and reflections, multi-coating aims to reduce that.
3. CPL is used to reduce glare, reflections and get more color contrast in the sky.
4. Using CPL on top of a UV or protector filter is not advisable. On wider lenses, it can cause vignetting, and the more extra glass you have in front of your lens, the more problems you will get.
5. Remember to remove your UV or protector filters when shooting at night to prevent ghosting or flaring. The best IQ you will get is when there is no filter in front of your lens.
6. Hoya and Kenko filters are same thing branded for different markets. They share some same model naming too, like the Pro 1 D range. Hoya HD = Kenko Zeta. Same. Kenko is slightly cheaper than Hoya. All made by Tokina using Hoya glass. I use Kenko mostly. Kenko is a private ltd and Hoya is a public company. There are talk that Kenko is either owned by one of the founders of Hoya or actually owned by Hoya itself.
7. The relationship between the 2 companies Tokina and Hoya is very tricky and very shaded. Both companies do not talk much about it. But there is talk that Hoya has a very big stake in Tokina (or owns it all). BTW, Tokina manufacture all Hoya/kenko filters using Hoya glass. Tokina is still a private ltd company, so there is no need to reveal who are the owners. But all over the internet it seems to point to the fact that Hoya owns Tokina, either that or the founders of Hoya owns Tokina. BTW, Pentax is also a subsidary of Hoya as well.
8. Price wise, Tokina is cheapest because they do not offer higher ranged filters. Kenko filters are almost identical to Hoya filters. Kenko is always just slightly cheaper, because Kenko stuff are mostly parallel imported into Singapore from China (where stuff are cheaper). Apparently Kenko Japan has a authorized distributor in China, and Kenko is really very popular among China photographers.