IMAP. POP3 is a one-way mechanism and cannot synchronise your mailbox between the server and your mail client. IMAP can two-way synchronise all your folders rather than just your Inbox, which means you can access your Sent Items, Drafts, etc. from any mail client or via Webmail with the right provider. You can also use IMAP to keep multiple mail clients synchronised with one account.
IMAP also sends all password information encrypted, so it's more secure than POP3 which sends passwords as clear text.