I use Thunderbird at home and Outlook at work.

I prefer Thunderbird though if I had to pick between the two.

Outlook Express

Before i used Outlook express and it is quite good and didn't encounter any problem on that one and now I switch to Thunderbird which is quite ok as of now!

outlook 2007 no doubt. It is the best email client. Its very easy to use and user friendly. I can do many tasks easily with outlook 07

Gmail!