For hiking...
2 x 1L Nalgene bottles.
All year round I charge up by drinking 0.5L before I set off on a hike. (Saves "carrying" it.)
In the darker half of the year I take 1L with me along with a Sawyer Water Filter and Squeeze Pouch. The rest of the year I take the aforementioned water filter and either 1L or 2L with me according to temperature and length of hike. The Squeeze Pouch can, of course, be used to carry additional water along with the Nalgene bottle(s).
I also carry a packet or 2 of Dioralyte (Blackcurrant flavour!) with me year-round to help with rehydration / electrolytes and use where I feel appropriate.
For camping...
As above, along with assorted Ortlieb Water Bladders and one of their ceramic gravity filters. I also have a Katadyn ceramic filter.