As I go backpacking 2-3x per month I am of course filtering water frequently. It's been an interesting ride over some time, tried a few systems out.
Sawyer Squeeze (the original largest 3oz version) coupled with a 3 liter CNOC Vecto dirty water bag has won out and stayed with me for a long time now. Seems to be the lightest weight, most reliable and utilitarian system out there; 6oz all-in. It will thread directly to one of my smartwater bottles I drink out of in the event of a water bag failure. It also easily backflushes when you insert a smartwater sports cap into the clean end. I have a little cup which is the bottom 3 inches of a smartwater bottle that the filter and rolled-up bag nest into. This cup is what I use to gather water from shallow seeps or potentially muddy water sources, when I don't want to disturb the fragile bottom layer of silt to kick it up into where I'm pulling water from. I am convinced that some of those who hated the Sawyer would change their minds if they used it with the Vecto bag. I hated it too at first when using it with the useless and fragile bags supplied with the filter.
If I am in an area with exceedingly poor water sources - highly polluted with say human pathogens - which hasn't actually happened on any of my trips yet, I have something called a Survivor Filter I got off Amazon for about $80. It's a 13oz three-stage filter with a 0.1 micron pre-filter, activated charcoal intermediate stage and a 0.01 virus level 3rd stage. It's a pump system and works fine. Not necessary in North America though. Just bought it for the giggles and gear addiction.
The one I hated the most was the MSR Trailshot. I think it's an ok system if you are into something like fastpacking or ultra running and need a quick, light compact filter. It just fails to pump frequently and seems to be a poor design. You have to fill the bulb completely to get a decent flow rate - which requires a combination of luck and skill and a deep water source; problematic in this neck of the woods. I think it took me an average of 20 minutes to filter 3 liters with the damn thing and my hand hurt pretty bad after all that "pumping".
I always carry chlorine dioxide tablets with my med kit in case of filter failure or exceedingly sketchy water sources. There's always boiling too if push comes to shove.