Something to bear in mind: when you use one spam filter for a while, it learns the sort of spam you get and will do a better job. When you switch to a new provider with a new spam filter, you have to train the new spam filter before it can be as good as the provider you left. (FastMail enables personal spam filters for improved results when it gets to 200 spam and 200 non-spam messages.)

Some find FastMail’s spam filtering better than Gmail’s, some worse.