My husband and I are biologists and shoot outdoors. He started taking pictures about 50 years before I did and has always been a fanatic about ALWAYS keeping a filter on the camera --a simple UV or haze filter. In simplest terms, it keeps dust off the lens.
And plant parts. And (I'm the botanist) it keeps twigs from scratching the lens. And it keeps my parka zipper --or the cloth-- from scratching the lens when the camera's being carried. A filter is terrific physical protection. Every time I slide down a hill (to be accurate, when a hill and I slide down) I think Thank heaven I've got the filter on. Generally, I have a lens cap on also --but it can be knocked off.
Tests I've tried don't show a lot of difference between no filter, a haze filter, and a UV filter (with several lenses the same size I got different filters each time, to see if there was a difference). If any of these suck light, it's minimal. I do have an 'ultra-clear' filter on the lens that I use in the lowest light (also the least haze or sky-glare conditions). There, I think it makes a bit of difference in making use of the light. The haze/ UV filters do seem to sharpen the colors a bit, as the filter-makers claim, in some conditions.
Because I knew I'd be leaving the filter on all the time, I got the best quality I could find under, I guess, about $50. The Ultraclear was a bit more, I think. Most of them are the best Tiffen makes. The Hoya's probably better but $20 difference? I dunno. One lens had a cheap filter on it (my husband's) and there WAS a difference.
As someone points out somewhere (not this thread), it is one more layer of glass. Someday I might want to take a filter off a lens --for one shot.
If you end up shooting a lot of sideways shots, the Canon hoods are a real pain. They only lock onto the lens with the big 'leaf' on top when the camera's held 'normally,' to take a picture that's wider than long. And hoods don't keep dust off the camera. I feel like a filter is a lot more protection.
While it's a lot cheaper to scratch a filter hastily wiping dust off it than to scratch a lens, mine seem to be quite washable. I rinse them in coolish water, rinse our hard water off with distilled, pat dry and make sure the inside is really really dry before putting the filter back on the camera. The air's usually dry here and I've never seen steam on the lens or filter after it's back on. (Shhh! I often dry the inside with rubbing alcohol before wiping or patting dry -- there are some horror stories here about using it on other parts of the camera and I'm not sure how good an idea it is.)
I'm with makphoto -- the dust and stuff I clean off my filters is frightening. And those coatings on the lenses look awfully soft.
Of all the filters mentioned, I'd put off the circular polarizer for a while. Judging from my results, there's a learning curve, and I have yet to run across a situation where it was needed. I ditched all the comparison shots with it (now I wish I'd kept some). Landscapes that were fine or needed minimal adjustment with the other filters were disastrous with the circular polarizer (even with f-stop adjustments for it). I suspect it would be a great help where there was sand or water.