best spam blocker

Your email provider should be filtering the bulk of your spam. I use Gmail on the iPhone and have virtually no spam.

Ditto with GMail for me.

As stated, your email provider should be doing this.
As for an App, NO app can integrate with another app (no plug ins, etc) so no app can work with the email client on the phone thus no SPAM app can be made (unless they build a fully functioning email client app themselves)

This s-ware says it can block spam on your iphone as well:
http://c-command.com/spamsieve/ if you use IMAP.

I haven't tried but may.

Patrice