The 870 is one of, if not the most, produced shotgun ever. It has a wide variety of aftermarket support and is easy to clean, take apart, and swap barrels. If you are looking for something that "just works" and don't need a shell extension, the 870 express is an awesome deal and goes on sale quite often at Sports Authority. If you want something with a little more, the 870p is about twice the price but comes with better parts and the shell extension.
Benellis are good too, but IMHO aren't worth the extra money. I've heard good things about the Mossberg 590 and 590a1. I think Mossberg, in general, is really a mixed bag based on the model.