What are the best water filters for camping, more specifically parasites such as Giardia?

Like others have said, any will work. The best choice depends on other factors: For yourself or a small group. Fairly clean water, or silty, or Mississippi mud. Is weight a concern. In North America, virus treatment (chemical additives) is a non-issue. Giardia and other parasites are easily captured by any good quality filter. I used to work in the woods of the Northwest and frequently just stuck my head in the stream and drank...until one of my co-workers had a section of bowel removed and spend a couple weeks in a hospital from good old Giardia. I use a PUR (now Katadyn) Hiker (Carbon/paper) and add a cheap pre-filter when it is likely I will be in muddy conditions. If I stay away from the Missouri River in Mt, I can get 2-3 years out of the $40 cartridges. I have used the ceramic MSR miniworks too. The ceramic can be cleaned often, the problem is that in muddy water it MUST be cleaned often. Great source of debate, but after a week on Old Muddy, I like just replacing the cartridge with a new clean one, along with my shoes, pants, shirts, and truck seat covers!

Katadyn has a very good reputation when it comes to water filters, although they do tent to be a little pricey. Most brands of filters are excellent at removing Giardia, but you could use tablets plus a filter if you're really worried about Giardia.

MSR makes a good ceramic filter. 0.3 microns has not made me sick yet. Katadyn is equally good, but requires replacement filters.

I've been using the Katadyn Hiker for a few years now and it works quite well. The only real difference I've seen from one brand from another, when they all claim to filter out all the bad stuff, is the ease of use. Cheaper brands are VERY slow to filter. For example 45min to fill on quart size container of the cheaper brands. The Katadyn Hiker I've used has filtered as much as 2 1/2 gallons in about 20mins of pumping when I've had to share mine in scout outings. As far as muddy waters concerned, I've always put a hankerchief around the pickup tube end, attached with a rubber band. This saves changing out filters frequently.

Bacteria and viruses can be killed with halogenization (chlorine or iodine), but protozoans cannot. Almost any filter will be satisfactory. The deeper in the wilderness you are, the cleaner the water will tend to be. This isn't suggesting that you should take chances, but a reasonably good filter will do about as good a job as a munical watewr plant will for purification.

The only time I have been ask to bring something to filter water was in Nepal and the suggestion was iodine a red chemical that kills 99% of every thing in the water. it seemed to work as I did not catch any thing. the down side of it it kills all the friendly Bactria in you stomachs well. giving one the runs on the return back to normality. I got mine from a proper chemist

you're safe with any of them, really. you're pretty much not going to find a filter that won't filter out what you need (at least unless you're headed to the 3rd World). the only real difference in water filters you could consider is whether you want a porcelain one or a carbon filter. if you camp at below freezing temperatures, then choose a carbon one, as the porcelain one can crack from freezing water inside. i prefer the porcelain ones because they can be cleaned off, whereas the carbon ones need their filters replaced far more regularly.