A recent post in Treehugger's Ask Pablo column asked just that. Pablo examined the Environmental Working Group's 2011 Bottled Water Scorecard, as well as a 2008 report they did on the health of a number of different bottled water brands, and the results were surprising.
According to the EWG, the best way to get your water is to filter it straight from your tap.
The tests showed that many bottled waters were chemically indistinguishable from tap waters; others contained "caffeine and pharmaceuticals (Tylenol); heavy metals and minerals including arsenic and radioactive isotopes; fertilizer residue (nitrate and ammonia); and a broad range of other, tentatively identified industrial chemicals used as solvents, plasticizers, viscosity decreasing agents, and propellants."
Turns out what's good for the environment is good for you too! Pharmaca offers a great selection of water filters, from pitchers to stick in your fridge to filters to attach to your faucet. Just pour your filtered water it in a reusable, BPA-free water bottle from SIGG or Earthlust and be on your way.