A good, general purpose, reliable, field-cleanable filter is the MSR Miniworks. Many ultralighters feel it is incredibly heavy, but most other filters are only 4 - 8 ounces lighter.

There are other good filters, but most descriptions of them begin, "As good as the Miniworks, except..."

Having said that, I'm going to try out a couple of Katadyn's offerings: the Mini and the Vario. I'll try to remember to let you know how that goes.