Best firewall gateway version of Linux ?

[Burn Alting]

Hi Guys,

Don't know if this will create a war, but I'd like to get some idea of the 
most common version of Linux used on as firewall/gateway system (eg 
Smoothwall, etc). I'm assuming the most common would mean the one of the 
better ones.

Also, how nasty is it to also make the firewall one's mail router (sendmail)? 
This is in the situation that all other nodes within the firewall are 
personal systems which may be running or not, and either Linux, Solaris or M$ 
boxes.

Regards

Burn Alting
burn at goldweb.com.au