Don't know about best, but www.fastmail.fm/help/overview_features.html is pretty cool. It is what it says, fast. No graphics, clean web interface, runs quick on anything, including crappy old hardware or phones. Opera recently bought them so if you run Opera on your phone it is especially nice. Very reliable, hardly ever down and on the rare occasions it goes down it's usually back up within minutes. You can hook up pretty much any client you like, of course. And they don't read your mail to serve you ads.