For my money, Flickr. For about $2 a month for a pro account you can use them as an online backup system (you don't have to make all of your pictures visible to everyone). They've got a great batch uploading tool, nice organization tools, and a good community. Join groups related to your pictures and add them to the pools, make sure to use good tags on your pictures.
I do wish that they integrated with more outside printing shops. Maybe in a future version.