For defense you can pretty much go with any of the big names, I personally am partial to Bushmaster. Depending upon what distance you will be engaging targets, you might want to look into an M4 or Flat-top so you can use some kind of optic as your primary sighting system. It's makes it easier to hit faster. Defensively speaking, if you need more than 16 inches of barrel, then an AR-15 is the wrong tool for the job. For less than 30 feet, I'll second the suggestion of a shotgun instead.