The free e-mail system with the best spam filters is clearly Yahoo!
They're also far less intrusive than either Hotmail or Google in terms of automatically enrolling you into other junk software / profiles etc
And more importantly - its easier to manage your profile and keep it private than it is with the others
AOL (I hate to say this) is a close second - its Spam filters are nearly as good, and the profile management is OK
Both enable you to use IMAP if you want - the AOL is easier to set up in that respect