I've got a CamelBack HAWG (military version--it's larger & of a better design than the brightly colored civvy version). I like it a lot. As others have noted, it comes with a 100 oz (~3 liter) bladder, & has attachment points for another one. As an additional/optional purchase, you can get a 70 oz (~2 liter) bladder, or even a 52 oz (I dunno liters), for variable needs.

At the time I got mine, the HAWG was their largest pack (still a smallish daypack, though). It has now been surpassed by a couple of models. I've used it cycling & sightseeing, & right now, it's riding in my truck as an augment kit the the stuff I normally carry in there. It's a great pack!

I also have a Camelbak "Unbottle", which is just the hydration bladder & hose in an insulated nylon sleeve. It can be inserted into any pack, or by attaching straps to the D-rings, carried on your shoulders (I use the shoulder straps from give-away/promotional nylon briefcases I've accumulated over the years.) It's great when all you need is water (like mowing or working in the yard, etc.

Both reservoirs (that's a nicer word than bladder) are the Omega style, which is a wide-mouth, easy-fill design.
Camelbak is, IMO, the best on the market. They pretty much originated the concept, as best I can tell.

David