Best in what sense? Optimal for the grain? Cheapest? Some compromise?
I use Sterilite/Rubbermade type tubs, with a strip of foam weatherstripping tape around the top to get a proper seal. The grain is stored in the bag, in these containers, in the basement. A bit cheaper than the Vittles Vaults, with arguably indistinguishable results.
I use a cheaper version from walmart. http://www.walmart.com/ip/Pet-Food-Stor ... s/10811477 It will hold 50+ lbs of grain, has an O-ring seal, and has wheels. I have three of them and am quite happy with them. They are somewhat fragile so you can't beat them around too much (or bounce them down the stairs like I did. ) but it shouldn't be any problem if all you do is roll it across the brewery.
I live in a dry climate, but have always done well with buckets from Home Depot. That said, I brew 10 gallon batches so a sack of grain only lasts me two brew sessions of 1.060 beer.
Can't beat the Vittles Vaults (mentioned in the third post). I had grain stored in one of those for 5 years (forgotten and rediscovered) convert. Plus they can be stacked and the side mouth ones opened while they are stacked. But there is a caveat. If there is too much weight on top of the mouth distorts when the lid is taken out and you can't get it back in.