I didn't vote because I don't use any of those....From what options are there the thread should be What Free E-mail Celirent do you use.... At the Moment I use GMail with Outlook Express and Thunderbird
My main email accounts are with Gmail (Renamed to googlemail in the UK due to legal reasons with Google and some other small company). The program I use to read them POP3 wise is Thunderbird.
My ISP mail is read with Netscape Mail.
Web mail is Yahoo.
I had the same Hotmail account since 1998 but let it die because the geniuses at Microsoft cleaned it all out on me with no warning, and it's much too slow for my tastes compared to Yahoo.
I dislike outlook. Its bulky, its back up method is sloppy and a pain. I hate messing with .pst files.
Thunderbird is an open source client which has many extensions like firefox, and it is handy. I use thunderbird for my home computer, and Groupwise client at work (since we run netware servers at my work).
I personally have not found an advantage for Thunderbird, but I use Outlook for it's many contact and time management features. Outlook has integrated task list, calender, flag email for follow up, etc. It's my "one stop" for tracking things to do, things I did, and such.