I considered five important criteria when using each reusable water bottle: filtering and filter capacity, materials, taste, ease of use and cleanup. These are all factors you should consider when looking for a water purifier bottle -- you'll want to purchase one that suits the activity you plan to use it for.
For instance, if I was looking for a filtered bottle to take backpacking, I wouldn't choose the Brita. I also wouldn't invest in the Grayl Geopress if I only needed a bottle for tap water.
Look for a filtering mechanism that filters out harmful contaminants, bacteria and viruses, and other unpleasantries out of the presumably non-potable water. Does the water bottle filter leave any particles in the water after filtering? Also, consider how much replacement filters are and how easy it is to replace them.
Most filtered water bottles are made of plastic or stainless steel. If the bottle you are looking at is made of plastic, make sure it is BPA-free. Also, make sure it is durable. Would a few drops dent the water bottle? If you are a hiker and need a lightweight bottle, is the material too heavy?
It's pretty obvious, but you want a filtered water bottle that leaves you with fresh tasting water. Specifically, you don't want any remnants of mineral odor or chemical tastes such as a chlorine taste.
Ease of use
Putting the bottle together should be easy. Inserting and replacing the filter should be simple. Also, consider how the water flows from the straw to your mouth. Do you have to tilt it a certain way to get most of the water out?
You want a filtered water bottle that is easy to clean. Check to see if the bottle is dishwasher safe. After use, what are you supposed to do with the bottle? Can you easily store it?