The best GUI, the best user support forum, the best configurable firewall, the easiest to use firewall (once users have learnt the basics)
New version fixes most bugs found with version 4.0 (talking about 1005 - 590). Anti-spyware module that is reliable and ranked. Plugins, content filtering.

The new Software Firewall King. ZA has become to instable and flawed with it's attempt to do everything version 6.5 up, ZA 6.1 is still good though.

This however wins the day. :)

Best firewall I have tested (ZoneAlarm, LavaSoft and Keiro).
Very nice GUI and help. Some features will be unnecessary for some (like me) but you can turn them off. They seem to only work in IE. Nice anti spyware even active scanning (easy to turn off). It is quite small with most things turned off. It starts at about 1,8 MB memory for me and grows (slowly) up to 30MB when using applications for a while, but when I open its GUI and close it, it goes back to 1,8 MB ... However most of the time it is quite small and I dont think it inteferes with games and such stuff (memory wise).

I use this firewall from the pre version 1 betas on and learned a lot during the years of using this baby. It's a very configurable program and can controle program access to the net with great detail.
Nice thing is the plug-in system witch makes Outpost very flexible.
Want just a firewall? Disable the plugins. Wants more? Take your pick from the plug-ins ot enable them all.
Latest added plug-in is an spyware module witch detectts spyware in real time.