I used to use Eudora. Since I moved to Thunderbird, I've been quite happy, and haven't looked back.
The only other program(s) I use are small programs that are more to scan my inbox to remove spam before having to download it into Thunderbird. Those 2 programs are Popcorn and nPopUK. They are portable and don't need installing, can handle attachments, are text-only email (so you don't get fancy HTML-enabled "flashy" email, but you also don't get 0-pixel web-bugs or possible javascript exploits either).
I'll scan my inboxes with either of those, then run Thunderbird to download/save/sort the rest of my email. I haven't seen anything better, but maybe one of those two small programs can offer something?