The Katadyn Pocket Microfilter is the gold standard, but pricey. (Got that from a guy on another forum who scientifically tested *all* of them for the gummint agency he worked for, nothing else even came close.) At a more reasonable price point, anything by MSR and Katadyn are pretty much equally effective. Mine is a Katadyn Hiker, it's served me well. I also have a couple quick 'n' dirty Aquamira Frontiers in my daypack and BoB for emergency use. If I had to buy again, I'd prolly go for a Katadyn Vario, for the fast-filter feature, and I've been eyeing an Aquamira Frontier Pro to toss in the BoB.
I have the smaller version of the Katadyn Microfilter... the Mini Pocket Filter. It's a third of the cost, and still has a 0.2 micron ceramic, cleanable filter element. The best thing about the ceramic filters is that you can open it up and let it dry completely after use, and/or scrape the dirty layer off. Hate the idea of the sealed paper/disposable elements that you store wet and dirty between uses.