In reply to Garston:
As others have said, Sawyer Mini is good as is the Sawyer Squeeze, if you don't want the faff of backflushing, are planning on using it in subzero temps and are worried about any moisture freezing inside the filter thus rendering it useless or only plan to use a filter occasionally rather than every time you're on trail then a Kayadyn BeFree could a good shout, though the obvious downsides are price, it won't fit on a standard bottle and the filter life is only 1000 liters (still a lot but in comparison to the Sawyer Mini's supposed lifespan of 378,541 liters it's not as "bang for your buck")
Personally I have both the Mini and the BeFree, the flow rate on the BeFree is far superior and if I'm out for the day/going for as light as possible that's my go to filter, if I'm out for multiple days though the versatility of the Mini is usually worth the faff of backflushing just because I know it would go on any plastic bottle if mine breaks or gets lost for some reason.