The filters I use, in order of frequency:
1. UV (on my lens at all times)
2. Polarizing
3. Neutral Density
4. Graduated Neutral Density
Many think #1 is a questionable practice and promotes a false sense of security because:
- it can cause a decrease in image contrast because of the added air gap
- lens flare from internal reflections is more likely with the added air gap
- the thin shards from a broken UV can gouge your front objective glass at the time it shatters, or when unscrewing the broken UV from the front of the lens.
If you always use a lens cap when the lens is not in use, use the hood and good camera handeling technique when it is in use, contrast will be improved, lens flare will be minimized, the lens will have an impact buffer zone, and you won't have to worry about breaking a thin filter and having it damage the big kahuna, front lens objective glass, right behind it.
We had a brief discussion in another thread about how the filter makers and sellers had to come up with a reason for digital shooters to buy filters that were no longer a staple like they were in the days of film. The discussion centered on a fairly well known camera store, Abe's of Maine, selling a 3 filter kit (UV, PL, FL) they were graciously reducing the price on, from $149.95 to just $49.95, with a lens purchase.
Lens filters have a large $$$ markup and often camera store salesman get spiff (extra pay) for selling a $2 filter for $49.95 or more.
UV filters improved
film images and were/are widely used by film photographers.
They can only improve digital images is a few limited situations, like at high altitudes (3000+ meters [10,000+ feet]) or when certain substances are caused by various light types to flouresce (wedding phoptographers sometimes see this with wedding dresses). They can, and do, degrade digital images to one extent or another, though the degragation cause by high quality (expensive) UV filters is more a technical consideration than one the human eye can perceive.
If you purchase UV filters, the filter makers and sellers will love you (cha-ching!).
The other thing about filters is: most filter effects can be duplicated with image editing software, with the exception of Polarized light and to a lesser extent, imaging flowing water so it has a smooth milky appearance, as is done with ND filters.