I like the Remington 1100 Tac-2 12GA Tactical Shotgun. It comes with an 18" barrel, magazine extension already installed, and has a simple bead sight which I like. The Tac-4 version has a 20" barrel for those who prefer them.
I don't have any experience with them but the Mossberg 930 SPX seems pretty cool too but I prefer the one without the pistol grip stock.