Assuming your looking at pumps and not semi-autos, the basic choices are 870 or 500 pattern guns.
If money wasn't an option then I would go with the Mossberg 590A1, you're looking at about $550 with ghost ring sights. The consensus seems to be the control placement is better than the 870 (don't have to dislodge the strong hand as much to manipulate the safety and bar release), and although the receiver is aluminum on the 590 it seems to be strong enough and has dual extractors vs the single one of the 870. The 590A1 has a heaver barrel and is constructed differently than the entry level 500's or a Mav 88 clone. In fact was just talking to a local PD yesterday and they want to replace their old 870s which are over 30 years old. Since rebuilding the old 870s would be about $350 they were looking at 590A1s but didn't want to retrain everyone on 500s from 870s.....but buying new 870 Police magnums would be significantly more than 590A1s and that even includes selling off their old 870s to cover part of the cost...they can't decide.
My agency has 870 Police magmuns with Knoxx reduced recoil stocks and rifled sights. I wanted a personal one so that I could train at my leisure at my personal range. I looked at new 870 Polices, new 870 Expresses that I could retrofit, old 870s I could rebuild, etc....decided on a Norinco 870 clone (a Hawk 982) that was already equiped with ghost ring sights and added a Knoxx reduce recoil stock with sling and replaced the sight screws with locktited socket headed hex screws. Given the materials used in the trigger pack of the new 870 compared to the 982, the machined extractor of the 982, the reported extraction problems with some of the new 870 expresses, and finally the price differences......the best bang for the buck was going with the 982, which was about $300 with everything, with the new stock being a third of that. At that price I got two......and while I've only put a couple of hundred rounds through both of them I'm liking the ghost ring sights better than the regular rifled sights on the ones at work.