Which e-mail client? You'll get no end of opinions on that, all based on personal misconceptions, not fact.
Personally I dislike Thunderbird: its not all that easy to set up, especially if you have multiple outgoing email accounts. Also development on it has stopped.
For most people the default Microsoft Live Mail that now comes on most PCs is adequate. It has a couple of PITA moments - it tries to get you to convert your email address into a Microsoft Live! account and get you to log in to Live!, but this can be easily disabled. Also the way it handles photo attachments isn't brilliant
There used to be lots of other clients, but nowadays its a limited choice to the above two, and a few paid-for programs such as "The Bat!" (which I like, but not enough to purchase)