I've seen camelbak-type packs (like this one), bottleholders of various types (usually hand/arm holder or belt, like the one @michael linked to), or even runners carrying plastic water bottles by hand. Possibly the most comfortable solution is not to carry water, but to have someone else carry it for you - hydration stations at marathons come to mind.
The best solution is highly subjective - some runners don't mind carrying things by hand, while others might find even a small camelbak to be chafing/uncomfortable. In the end, the runner is constrained by the amount of water that he/she wants to carry, personal comfort, and the amount of money that he/she is comfortable spending. The possibility of reuse for other activities like biking or camping can be a consideration in selecting a solution as well.