After using Yahoo! Mail, Hotmail and Gmail, I believe Gmail is the most efficient of the three.
Although, you could probably say that Yahoo! Mail is more reliable, Gmail has sooooooo much space that you could probably start using it as a storage service....well, for yourself of course. Gmail doesn't append any ads to outgoing emails like Hotmail does, and has limited ads in the browser window. For Yahoo! and Hotmail, Hotmail especially, more than half the bandwidth used by checking your mail is loading stupid ads, whereas Gmail only has small text ads beside the email window.
Many a time ago, Hotmail accounts were one of the only ways you could connect to MSN Messenger, so that was the only reason I signed up for an account. But since the .Net Passport allowed other email accounts to obtain an account to sign into Microsoft Services, it just meant I could move on to bigger and better things.