Gmail. Easy to use, easy integration with Google Talk, Blackberry applications for Gmail/Google Talk, and most importantly, awesome POP implementation that allows me to receive mail from all my accounts (ISP, personal domain, etc) in my Gmail inbox. Then I use POP access to backup my Gmail account to Microsoft Outlook from time to time. And my Blackberry only needs to sync with one account, while Google does the heavy lifting condensing all my accounts in the background. I used to be an Outlook fanatic, but when I found out Gmail could pull email from multiple POP accounts, I switched and never looked back. edit: I forgot about Gmail's excellent spam filtering, which it applies to all my accounts.
I suppose you might be able to do all that with Yahoo, but I hate their messenger, and everyone I would want to chat with has Gmail/Google Talk anyway. There's also kind of an "AOL" stigma that goes with a Yahoo account.