Sawyer Mini. It's inexpensive, reliable, and fairly quick. It can also be backflushed in the field. It also allows you to use just about any water bottle you want. My only tip: take along a gallon-size ziploc bag. I found it really hard to fill the "dirty" water bag in a flowing stream (the easiest fill is if you can catch water falling vertically.) You can always use the ziploc as a bucket to fill and then pour it into the dirty water bag.

You can't attach the clean bottle to the filter, which sometimes makes it a bit of an awkward operation. If you want a filter where everything connects, is easy to fill and backflushed, and very light and compact, then get the Platypus GravityWorks 2L Bottle Kit and a 1L Playtpus bottle. More expensive, but really easy to use.