Outlook Express, because it's always there.
I have Microsoft Office and thus have Outlook, but I'm used to Outlook Express, and and my feeble mind hasn't so far figured out why Outlook is better.
By the way, I can let you Outlook Express users in on a little secret. You want to backup your mail or transfer it to another Outlook Express? Couldn't be easier. Select all your messages in your inbox folder, then drag and drop them to a regular folder. All the messages will appear in the regular folder as .eml files. To import them into a new Outlook Express, just select the messages in the regular folder and drag and drop them into the new Outlook Express inbox window. But you must promise not to tell where you heard this
By the way, I don't need synching, and I don't need backups. I have several XPs installed amongst my two computers and I let Outlook Express in several of my XP user accounts pull down the mail. But I tell Outlook Express not to delete the mail on the server after downloading. I only allow one of the OE accounts to delete the mail on the server, and only after 20 days.