Gmail is my favorite. Not only do you have a ridiculous amount of storage (almost 5 gigs now?), which you can use as storage with a simple FireFox plugin.
The spam filters on it are incredibly effective.
Gmail also offers features that you have to pay for at Yahoo, like for instance the ability to use a POP mail client. Gmail also allows you to use IT as a POP mail client, receiving mail from other mail accounts as well as being able to send mail as those other accounts.
I especially like the way it stores email conversations almost like a forum thread, so you don't have 100 emails stacked on top of each other.
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