I keep Hornady Tap in my AR. Great round, accurate, performs well.
Even better is the Hornady Urban Tap, designed for police use in urban scenarios. Tough as hell to find though.
Seriously though, if you want your AR to be a defensive tool, pick a load that will do the job. Tap is a great round.
http://www.hornady.com/store/TAP-FPD-Bullets/