I can't vouch for what's best, but lots of searching has turned up some good ones, of which here are a couple:
"ASL University", http://www.lifeprint.com/asl101/index.htm, is a site that tries to provide " a college equivalent ASL curriculum online that can be used in fulfillment of ASL course requirements in high schools and colleges". I really like Dr. Vicars's practical approach and will be ordering his text shortly (like, after I finish posting here...).
Karen Nakamura's Deaf Resource Library (http://www.deaflibrary.org/) has MANY links to useful sites, not just about ASL.
