What makes Gmail the best webmail client?

People often say that gmail is arguably the best email webclient [1]. I realize that it offers decent storage and anti-spam, but I find it hard to believe that both of these features are so much better than all their competitors.

I've been using Gmail for many years now, so I don't really remember other services. To me, gmail seems cluttered and clunky.

What about Gmail makes it the best?

[1] "Gmail is arguably the best web-based email client out there"

via http://techcrunch.com/2011/04/15/greplins-chrome-extension-now-makes-gmail-search-infinitely-better/