The best room to use as a fallout shelter is the one that is farthest from the source of the radiation. Remember the key principles of radiation avoidance:
1.Time
2.Distance
3.Shielding.
Put as mush of all three of these between you and the radiation as possible. I'm assuming you're talking about fallout from some nuclear event. A room on the ground floor should do nicely, as long as it is sealed from the rest of the house. Anything you can put between you and the walls is beneficial. I don't know that I would use barrels of water if you plan on drinking it though. Don't forget that fallout could enter anywhere there's a draft. A safe bet for protection is sand bags. They're cheap and easy to find. Stack them against the exterior walls. I'm no radiation expert but I hope this helps.