Well, that would be Comodo. If you have read any of the more highly respected comparisons, you will see it on top more often than not. Next to that, the stripped out free versions of Jetico and Online Armor are pretty good, though they tend to be beaten by one or two premium products.
Sunbelt's firewall is actually pretty poor in some very thorough tests. It passes the old GRC leak test, which is fairly pointless in practice, but it seems that it's not that hot in other areas. Check out its rating from Matousec for example.
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