What is the best type of dust filter to use with a desktop PC?  size, material, maker?

dust enters via every indraft fan -- some come with a filter that goes in front of the fan ... see aftermarket for other such filters [they do have to be periodically cleaned, but this can be extended by using shopvac from the outside while computer is not running].  -- grampa

I live in a tropical environment so I know dust and I know dust that gets moist inside the CPU. Generally, I don't recommend installing dust filters. It blocks air passage and restrict heat dissipation. What I recommend is an annual blast of compressed air to the innards of the CPU. Try to get every nook and cranny especially the PSU and the fan and heat sink of the processor.