I currently carry a vacuum-insulated Klean Kanteen (20oz). Hydroflask makes a 24oz. version that is somewhat heavier as well -- the slightly taller bottle is nice, but the lid isn't as good as the Klean Kanteen variant (the two caps appear to be interchangeable).

If you regularly encounter dodgy water, something like the Grayl is really good, but capacity is limited (16oz.) and the bottle plus filter cartridges are quite expensive.

Edit: how do people feel about water bladders?