Best is a tricky idea to really pin down, but for the purposes of this list I've tried to go by a few criteria such as weight, reviews, reputation, and features to judge which are the best backpacking water filters.
Here are a few of the best backpacking water filters that fit the criteria.
Best Backpacking Water Filters List
SteriPEN Adventurer Opti - If you're looking for the new great thing I would highly suggest the UV water purifiers made by SteriPEN. The Adventurer Opti is one of their newer, lighter designs, but all the SteriPEN models work on the premise of using UV light to neutralize all the nasty stuff in your water. Keep in mind that it doesn't filter the water, but anything from a spare shirt to a bandana can be used as a crude sort of filter. The one downside to something like this is that you'll end up needing to replace batteries every now and then. If new and innovative is what you judge the best backpacking water filters to be then check out the Adventurer Opti.
MSR Hyperflow - A classic pump filter made by MSR, who have been making water filters for a while now. The Hyperflow won a Backpacker Magazine Editor's Choice Award in 2008 for best backpacking water filter and is the second lightest pump filter on the market, only 11 grams behind first place. An all around serious contender in the best backpacking water filters category.
Katadyn Hiker Pro - From a well respected brand in all kinds of water filtering technology, the Hiker Pro is another pump filter only slightly heavier than the Hyperflow. It gets a lot of very positive reviews and is an all around great choice for best backpacking water filters.
Platypus Gravity Works - If you're looking for something a little different in terms of water filtering than the Gravity Works might be for you. Gravity filters are generally only used while you're stationary, so if you break for lunch at a water source or prefer to have all your water filtered in the morning so you're ready to go the next day, a gravity filter might be a good choice for you. The Gravity Works is the lightest gravity filter on the market and Platypus is well known for their water carrying devices.