Rev2.org has a list of 10 Web 2.0 companies where "the criteria were as simple as (a) they had to be a new startup -- Valley-based or not, but most are anyway, and (b) they had to be looked at as a Web 2.0 company (i.e. no eBay or Yahoo! or Craigslist)."
They are pretty much the ones you'd expect: MySpace, YouTube, Facebook, Wikipedia etc. Sid Yadav admits to overlooking Lastfm but this is easily done -- I'd have have overlooked NetVibes and perhaps Meebo, for example.
If you really want to start a fight, you have to go for a top 20.....