FLICKR - would be my choice, simply because it:
- Has a good balance of web 2.0 magics(ajax, tags, community, foaf). You get all the familiar tools that have become standards - in an easy UI.
- Has a Deep User Experience- Flickr is more than album sharing. It's a living playground where people interact through pictures and their related stories. You delve into Flickr emotionally.
- Stays Slim- no point in being a bloated "all-in-one" website. The ones that managed to combine elegantly a slim design with maximum experience and stickyness are the pillars of Web 2.0 (digg is another one, but appealing to a smaller userbase).
Why not MySpace???
Only because the search there is so crap, in a way that leaves any above-average user very frustrated.