I use Eudora OSE. Part of this is because, back in the early '90s when I was the Internet Services Administrator for a Federal agency well-known for its extreme technological prowess, we decided to standardize on our email clients agency-wide and did a comprehensive study all the email clients available at the time. We found old Qualcomm Eudora to be the best balance of compatibility, features and ease of use. I've used it and its open-source successor ever since.
Eudora OSE is basically a user interface overlay atop Mozilla Thunderbird that preserves the look and feel (to a large extent) of the old Qualcomm product. It's freeware and open-source, and that's always nice. It is also VERY easy to transfer all the data and settings, including all saved email and the folder structure in which you retain it, from one computer to another.
But all the email clients do most of the same things, just some do some in different ways. I think most people still use Outlook or Outlook Express (or its successor, Windows Live Mail), and it's not a bad product, but I just don't like it much myself. It's purely a personal taste thing.