MailWasher, because it only downloads a small part (customizable by the user) in text mode and no attachments to determine if the email is spam. Once spam is identified it is deleted from the server and then you download the remainder using your email program. It is highly customisable and configurable. The free version only handles one email account (you could elect to have your email forwarded to one account in the other email accounts), for multiple accounts the Pro version is reasonably priced.
There are other free anti-spam tools but not many that delete from the email server, so you still have to download all that spam to determine if it is spam or not.
