Well, of course, everyone has their preferences, and that's fine. But I do want to keep this discussion going.
First, Ofoto (now Kodak EasyShare) and Shutterfly are great services. I use Shutterfly a lot for client proofs and think they do a good job. But I don't think of these services the same way I do flickr, which I also like.
Flickr is the easiest way to take a shot I've just captured and share it with others, especially those who are my friends and family. I like the tagging system, uploading tools, RSS output, etc. Also, the site is just plain enjoyable to use. Browsing and looking at other photographers' work runs the entire spectrum from impressive to depressing.
So even though other services, including .Mac are noteworthy, in general I feel flickr is the most innovative of the bunch and comes closest to a Web 2.0 application.