Most Powerful Security Tool
It's Between Your Ears
Why?
Because "Security is man-vs-man and humans are intelligent." - more about this here: What is it that makes security hard?
I am strong believer of process integration when it comes to security - more about it here: http://msdn.com/securityengineering
And here are some tools to support the process:
Threat Analysis and Modeling - http://go.microsoft.com/fwlink?linkid=77002
FxCop - http://www.gotdotnet.com/team/fxcop
FindStr - Security Code Inspection - First Look For What To Look For
Fiddler - http://www.fiddlertool.com/fiddler
NETMON III - http://blogs.technet.com/netmon/default.aspx
Cheers