All of this is meaningless without reference to lens design and focal length. With many zooms and with all retrofocus wide angles, dust or damage on the front element is serious because the lens is in effect a rear prime group focusing through a negative front group and at small apertures the front element will be 'sharp' enough for marks to show in-picture. With telephotos, the front element is generally the prime lens in the design (it does the imaging) and the rear groups serve to reduce back focus distance by refocusing the image to a closer focal plane. Marks on a telephoto front element rarely show and you can take a chip out of the side of a 300mm f2.8 without noticing. Unless it happens to catch sunlight or something and throw up a problem. You can shoot with a cracked fluorite element in that kind of lens, wide open, and get a normal looking pic. You can even stick carpet tape over the middle and get a normal shot.
To confuse matters, some lenses have a sacrificial front element, especially fast apo teles with internal fluorite elements. This element either has minimal effect, or none. It's there to seal away an expensive soft element which can discolour if exposed to UV light and air (old apo fluorite lens elements go yellow). So chipping away at such an element or cleaning it with wire wool will just turn it into a kind of softener.
Fine scratches, cigarette smoke film, grease film or dust all degrade images badly. A really perfect, factory fresh, new lens will often outperform the same lens as found in a studio after a decade of use and cleaning - and checking under a magnifier, or hold the lenses up to light and looking through them, quickly shows why.
As for filters, the reverse situation applies with wide and tele lenses. A badly made UV filter will not lose sharpness on a wide-angle shot, for the same reasons you can safely shoot through glass windows with a wide-angle - especially when stopped down to f/11 or so. The distortions created by the filter are local, and are seen as local loss of sharpness. But try shooting through a non-plane-parallel UV on a 300mm f2.8 and you may not even be able to get focus. The entire ray bundle is affected by the poor filter. Same with a window. Try this - find a window with some old, wavy glass (maybe a greenhouse, I live in a house with 200-year-old glass and most of mine is very wavy but modern houses often have almost perfect window glass). Take a pic using a 28/18mm lens about a foot from a window pane at full aperture looking out. Now try the same with a 300/200mm lens. Then see the effect of moving the lens right next to the window in both cases.
Bottom line - it pays to keep all lenses as clean as possible, and never to clean them by rubbing with a cloth. Good practice means fitting a UV or protector. For short focal lengths and smaller apertures (i.e. 18-70mm kit lens) this does not need to be super-quality, but plastic/resin ones and very cheap glass in plastic rims are worth avoiding. Marumi, Izumar, Kenko, Hoya, Sigma*, Soligor, Vivitar and the German makes are all OK. I'd avoid Jessop, Tiffen, Cokin, Singh-Ray, Photax, own-brand chain store, Raynox etc. For fast lenses (85mm f1.4) or long lenses (70-300mm SSM G) the best quality you can get is essential - even more so with 300mm f2.8, 500mm f4.5 etc designs. Sony, Zeiss, B+W, Rodenstock, Heliopan, Minolta, Nikon, Hoya top line, Sigma*, Olympus, Canon, Contax, Rollei.
*Sigma - not sure - they may be good or very good. They are certainly well made and priced.