In your opinion what is the most powerful free firewall? I am using zonealarm free right now

In my book that is the best I know. Obviously there is Kerio and some other free ones, but Zonealarm does what one expect it to do

Even though it is no longer being developed or supported, I have found Sygate Personal Firewall not only to be a good firewall, but it also has useful logs, a backtrace and who-is function.
Comodo is also very good, but some of the functionality seems a bit obscure for the average user.
Regards,
John

I am using Kerio.

In my opinion, Comodo is a very sturdy and light firewall.
Recently, I have been having problems with ZoneAlarm and several times it has had launching errors, so I uninstalled it and switched to the newly released Comodo 2.3.6. It is lighter on system resources than ZoneAlarm and just as powerful in my opinion. (The interface is also more appealing to me).
But it's all a matter of opinion ZoneAlarm, Kerio, Sygate, and Comodo are all excellent firewalls.

thanks for the input. Do kerio, syagte, and comodo allow locking of host file?

I'm not quite sure about that particular feature in all of the firewalls, but if you have Spybot, HostSecure, or any other Hosts manager (well at least most of them) have the ability to lock the hosts files.