Fence sitting time...
If you already use Opera (or care to try it) the built-in email client is really very good. Simple interface, connects very nicely to POP3 or IMAP emails, and is part of an excellent browser.
If you don't want to use Opera for any reason, then Thunderbird would be best. Cross platform, stable, easy to use and you can add all sorts of little tools to make life easier. We use it at work because we can have the interface and spell check in any language, something that can only be done with Outlook if you have the necessary (paid) language packs.
TTFN